Good question this. I always thought it was to make fancy patterns in the sky, so you'd know which way the wind blew - in case you couldn't work that bit out by licking your big finger and sticking it up in the air.
Seriously though, the planes are high up in the apple pie burning their hot jet engines bringing Auntie Maisie from Adelaide to give us our prezzies/souvenirs. Up there it's cold enough to freeze your [appendages] and the air's thin enough to suck your head off if you're daft enough to stick your head out of the window.
Ergo, white trails - they look pretty in the sunset, don't you think, all feathery, wispy and such?
That's the easy answer. Are you ready for the scientific one? Right, here we go then...