I've noticed that Boeing 737-800's cockpit has a 3-position landing gear lever. During takeoff, the pilot moves the lever from DOWN to UP and then to OFF position. What's the point of the OFF position? Why not simply have UP/DOWN positions?
When UP or DOWN is selected, the hydraulics pressurize in the appropriate direction.
Once the gear is up and the mechanical locks are keeping the gear locked in place, OFF can be selected to disconnect the hydraulic power.
With the gear handle in the UP position, the retraction side of the landing gear actuators remain pressurized. The gear will stay in the full up position, however they will not be hanging on the mechanical locks designed to hold them in the up position. They've simply hit the full up mechanical stop.
When the handle is placed in the OFF position, hydraulic pressure is removed from the retract side of the gear actuators, allowing the gear to settle down and onto the mechanical uplocks, which are essentially hooks that just hold the gear up.
There are a couple reasons why it's beneficial to select OFF. One is that if they remained pressurized, then the gear are essentially hanging in space, at a mechanical stop provided by the actuator. Hydraulic actuators are delicate components, and these weren't designed to hold landing gear for an extended time in the retracted position. Imagine the loads imposed on the actuator in turbulence for example, as the gear are loaded vertically with each bump. Removing hydraulic pressure allows the gear to hang on the mechanical uplocks that were specifically designed to hold them, without putting additional wear on the actuators.
The other reason is that gear actuators are major users of hydraulic fluid volume, and the actuators and the lines that supply them are a potential for leakage. If they were to remain pressurized for a several hours long flight, that increases the amount of pressurized lines and components that can leak and cause a complete loss of the hydraulic system.
Note that if you look at more recent Boeing airplanes such as the 777, 787, and 747-8, you will not see an OFF position on the gear handle, only DN and UP. The OFF function is done automatically on these models. After you select UP and the gear are all fully retracted, there's a short time delay and the system automatically depressurizes the retract side of the actuators. It removes one After Takeoff checklist item for the pilots to do.
To answer some of the questions posed in the comments: If you selected OFF while the gear were still in transit in some intermediate position between up and down, they would simply freefall into the down position since you've removed the retraction power. If there's a leak in the landing gear actuator or its associated hydraulic lines, it would cause a loss of that hydraulic system, and all other users of that system would be failed. The 737 has two (and a half) hydraulic systems. The half is a standby hydraulic system that's very limited in its function, but I digress.
I worked on the 737 Landing Gear system design in the 90s and can try and answer the question from memory. All Boeing Commercial airfraft used mechanically positioned hydraulic selector valves until the 777, 787 and now the 737 NG starting about 3 years ago (now electrically commanded). The cockpit handle drives a quadrant which drives a cable loop to a 3 position spool valve in the MG wheel well. So it can't be automatically depressurized by an electronic control system. Additionally there are several reasons to depressurize it in flight, although that question was heavily debated during the 777 system design.
The Nose Gear hydraulic pressure line which connects to the selector valve in the MG wheel well runs through the pressurized cabin compartment below decks. If it sprung a pin hole sized leak at high pressure it would eventually mist the cabin air making it uncomfortable to breath. Skydrol is nasty stuff to get in your lungs. Been there. So depressurizing it makes a leak less likely (in theory) and if it did leak at low pressure it would not form a mist.
The landing gear hydraulic supply is common to other flight control systems and so the hydraulic pressure droops from system fluctation would add fatigue cycles to the landing gear system if the valve remained in the UP position in flight.
On the other question; leaving the gear handle up in rough weather is news to me but there could be some airlines that chose to do so because the nose gear has a tendency to unlock in flight if it bounces hard enough. It doesn't have an uplock hook but instead has a reinstating overcenter lock brace. So I could see it being done. Also, if both Main gears are deployed at full cruise and do not come back up, this is a big deal. On the Seattle Honolulu run, it would be a tough swim due to the drag fuel burn issue.
Note: On the 777 there are two selector valves. One for the nose and one for the main. So the line running to the nose is pressurized in flight. Or at least it was when I worked the system in the early 90s.