Why is GMT used in aviation?

Answer 1

When pilots are given a clearance to cross a fix or leave an altitude at a certain time, especially when outside of radar coverage, such as over the North Atlantic or Pacific, the pilot can take comfort in knowing that air traffic control and, indeed, every airborne flight all over the entire world is using the same exact time reference to the second. If they did not, planes would crash into each other with great regularity due to fact the one was using a different time than another.

The railroads learned this the hard way in the 19th century. Before the advent of standardized time, each railroad company kept their own time, usually based on the local time of the railroad company headquarters. Many switch operators had to keep track of as many as seven different times. This caused a great deal of confusion and trains frequently ended up on the same track in opposite directions resulting in many horrific accidents.