It's hard to say, but the answer is probably the same reason why we learn music, P.E etc.. in our muggle education. Different people have different aptitudes and passion, and a school should attempt to nurture these in students. This is even more important when the academic institution in question is one of the very few wizarding institutions in Europe.
That said, knowing how to ride brooms has practical applications too. Those who are unsure about apparition may use brooms for traveling.
"Er - splinched?" said Harry.
"They left half of themselves behind," said Mr. Weasley, now spooning large
amounts of treacle onto his porridge. "So, of course, they were stuck. Couldn't move either way. Had to wait for the Accidental Magic Reversal Squad to sort them out. Meant a fair old bit of paperwork, I can tell you, what with the Muggles who spotted the body parts they'd left behind....."
Harry had a sudden vision of a pair of legs and an eyeball lying abandoned on the pavement of Privet Drive.
"Were they okay?" he asked, startled.
"Oh yes," said Mr. Weasley matter-of-factly. "But they got a heavy fine, and I don't think they'll be trying it again in a hurry. You don't mess around with Apparition. There are plenty of adult wizards who don't bother with it. Prefer brooms - slower, but safer."-Harry Potter and The Goblet Of Fire
There have been plenty of times in the books where brooms were used for transportation or escaping. A few instances of these:
1)Brooms were used in the Order Of The Phoenix for traveling to 12 Grimmauld Place.
2)Harry and Dumbledore flying to Hogwarts in The Half Blood Prince.
3)The Battle of The Seven Potters
In the wizarding world, luggage is never a problem. You can always make it lighter.
So overall, making Flying Lessons compulsory for the first years is a pretty good idea.