I can understand that we have control zones at busy airports to streamline the flow of landing and departing traffic. But in Control areas of many airports i have seen that routes go straight through the control area upto control zone. I mean no noticeable change occurs in route geometry. I can't understand what purpose are these control areas serving the air traffic services personnel? In other words, are'nt control zones alone enough to streamline landing and departing traffic at any airport? If not, why not? I am really confused about this.
The main purpose of controlled airspace is NOT to "streamline" traffic, but to protect traffic. Control areas are not there to "service air traffic service personnel", but to enforce certain rules upon the pilots.
In and around busy airports, the density of aircraft in the sky is relatively high, meaning there would be a relative high risk of collision if everyone were just flying around randomly. To mitigate this risk, various types of controlled airspace is established (control zones, terminal areas, control areas etc.)
Within controlled airspace, aircraft are subject to an ATC clearance, which more or less eliminates any risk of collision. Crucially, outside of controlled airspace, pilots can, essentially, fly pretty much however they want, and the only way to not hit another aircraft is if you happen to see it out the window.
Control areas are not just established around airports, but in any area where the density of air traffic is high. Typically, this means pretty much everywhere at higher levels, and around major airports at lower levels.
ATC’s most fundamental job is to “separate” aircraft, i.e. keep them from hitting each other. The airspace where they are able to provide this service is called “controlled airspace”, and it can be divided into three basic types:
Control Zones designate the airspace controlled by the Tower, which is just a few miles wide and a few thousand feet high to cover the airspace they can see (in good weather), i.e. the traffic pattern.
Airways, in contrast, exist to protect en route traffic up high (far above any Control Zone) and mostly form straight lines between VORs. Since the most common place to put a VOR is at a major airport, though, the airways will appear to go directly to a Control Zone. But they actually go over the Control Zone. Note that many airways do not have complete radar coverage, so separation may be done procedurally.
Control Areas (TCA/TMA) are the bridge between the two. They are generally an inverted wedding cake around a radar site, and the most common place to put radar is at a major airport. Their job is to allow ATC to protect aircraft moving vertically by vectoring them (or through the use of SIDs/STARs) around each other laterally, and to exclude other low-flying aircraft if needed due to traffic levels.
Any airspace outside these three (which is most of the planet) is usually uncontrolled airspace, where means aircraft must depend on Big Sky Theory to protect them, because ATC can’t.