For example a Cessna Grand Caravan doesn't have a pressurization system, but a Phenom 100 is pressurized, so what are the main reasons a manufacturer has to build an aircraft with a pressurization system: altitude, speed, etc?
The main reason is that it's simpler than having everyone wear supplemental oxygen masks. There is no specific requirement for pressurizing an airplane but there are requirements for supplemental oxygen as per FAR 91.211
So by pressurizing the plane to under 15,000 ft you can fly over that altitude and not need to supply the passengers with supplemental oxygen for the duration of the flight. There are unpressurized piston planes that fly well into the flight levels but are required to carry supplemental oxygen and everyone aboard needs to use it, it's a look some people in general aviation don't mind,
On a slight side note the engine does have an effect on this choice when it comes to light singles. Pressurization systems on turbo props/light jets pull their air from the compression stage of the turbine which is already compressing air so no extra compression hardware is needed. If you have chosen a piston engine for your design you need to drive some form of pressurization system which can add weight and complexity. There are a few pressurized piston planes out there like the Mooney Mustang, Cessna 210 (came in both pressurized and un pressurized) and the Piper M350 (the only one currently in production).
An aircraft is never "Required" to be pressurized. The only reason to pressurize an aircraft is profit.
No one would ever buy an un-pressurized Phenom 100 or B777, but there is nothing stopping you from building one.
Pressurized aircraft are built because they can generate a profit for the manufacturer. Any aircraft can be made with pressurization but it comes with a price. If marketing determines it will sell, they will build it with pressurization.
You can look at pressurization as simply a comfort option. A comfort option that comes with a price.
There have been many cases of aircraft being built in both pressurized and un-pressurized versions.
The Cessna 402 and 414, Cessna 335 and 340, Cessna 210 and P210, Piper PA-31 Navajo and PA-31P Pressurized Navajo are all examples of the same aircraft built in both pressurized and un-pressurized versions.
In all these examples the un-pressurized aircraft can fly well above 20,000' but is generally operated at lower altitudes due to the inconvenience of using supplemental oxygen.
Generally, the operating altitude of the aircraft determines whether the aircraft is pressurised or not.
Regulations require the pressurised cabins to maintain certain perssure at the maximum operating altitude. For example, FAR 25-841 Pressurised cabins requires the cabin pressure to be maintained at 8000 ft at the maximum operating altitude of the aircraft.
Thought there is no set altitude of operation for the aircraft to be pressurised, it is required to provide supplemental oxygen if the pressure altitudes are above 12500 ft:
Basically, the aircraft designer has to decide between pressurising the aircraft (which increases weight and complexity) and having a non-pressurised fuselage and operate at a lower altitude, where the performance may suffer (or provide supplemental oxygen). It is a tradeoff and the decision usually depends on the aircraft design requirements. Note that pressurised aircraft also have to be provided with supplemental oxygen above FL250.
For example, the Cessna Grand Caravan operates at a cruise altitude of 12,000 ft, has an unpressurised fuselage, while the Phenom 100, which operates at a higher altitude has pressurised fuselage.
The requirement for a pressurization system is dictated by the operating altitude. With increased altitudes the atmospheric pressure diminishes and so the concentration of oxygen available.
T o have an idea of what is consider a "critical altitude" for an average human we can have a look at FAR 91.211. It requires to use oxygen when flying above 12500ft (3800m) for more than 30 minutes, and in any condition about 14000ft (4300m).
The CS-23 certification requirements, which applies to aircraft 12500lb (5760kg) state the following for pressurization systems:
These are the requirements a different aspect is what a manufacturer selects for its design which be influenced by other factors such as:
Taking a broader view.
If the "partial pressure" of oxygen (that is the overall pressure of the gas mixture multiplied by the propotion of oxygen in it) in the gas mixture people are breathing drops too low then people will suffer serious and potentially life-threatening medcial problems. We need to prevent that happening. There are several options for preventing it each with different disadvantages.
One option is to simply fly low enough. This is a common soloution for low speed travel but it doesn't work well for high speed flight. It may also cause issues if you need to fly over mountains.
A second option is to have everyone use supplemental oxygen delivered through some form of mask (possiblly a full face mask, possiblly a non-sealed mask like the one in Daves picture). This is an option for military planes and some GA stuff but I don't think it would work too well for airliners or buisness jets.
The third option is to pressurise the cabin. This allows you to fly at high altitudes where fast flight is practical while providing an environment where people don't have to tether themselves to oxygen masks. The downside is that your fuselage is now a pressure vessel and worse it's a pressure vessel that experiances frequent pressure cycles. As a result pressurised airplanes require far more careful design, construction and maintinance then unpressurised ones.
A fourth theoretical option would be to use an oxygen enriched atmosphere in the cabin. Afaict this is not used in avaition, I don't know exactly why but I would guess a combination of fire risks and the difficulty of supplying enough oxygen.
Aircraft do not need to be pressurized.
Above about 12,000 feet humans need additional oxygen.
Above about 50,000 feet any humans inside the aircraft need to be wearing pressure suits.
There is no "requirement" for any aircraft to be pressurized.