Because it reflects on the character. When an overweight guy named Porkins goes into combat without attaching his strap, I’m thinking “what’s that fat, lazy slob doing going on a combat mission.” The other pilots are your typical in shape fly boys and they all attached their strap. So here is the one overweight guy. Not only is he overweight, but his name is Porkins. And not only is he overweight and named Porkins, he’s the only one who didn’t attach his strap. Stuff like that doesn’t happen in the movies by accident.
I read an interview with William Hootkins (the actor who played Porkins), some years ago (he died in 2005), in which he was recalling filming “Star Wars”. One specific thing I recall is that the orange flight suit costumes that they had were “one size fits all.” Not surprisingly, they didn’t fit him; the costumer split Hootkins’ flight suit up the back so that he could fit in it.
As a result, he had to be certain not to slump too far forward when he was blowing up, so that the camera didn’t catch the T-shirt he was wearing under the orange suit.
Oddly enough, this isn’t terribly implausible. Apparently they encountered turbulence when going through atmosphere, and since The Force alone knows what you might encounter in any given battle, it’s not a bad idea to protect your noggin from getting slammed around. Even just exhaust fumes from a cap ship that size could do. Assuming they have exhaust fumes.
Second, apprently there were explosions in space which could knock ships about. We know at least some cap ship guns fired extremely destructive shells (turbolaser or not), and while the shockwave would be small (limited to the immediate matter of the shell) a helment could keep you from breaking your skull open. After all, if you lost consciousness you are poretty much dead, so most competent militaries will enforce that kind of uniform.
Also, other sources make it clear that fighters could be non-fatally damaged. Some pilots wore spacesuits they could eject in for hopeful rescue.
That wasn’t Porkins. Biggs had gotten Porkins killed minutes before that. “Stay on Target” was Gold 5. I’m sure he’s got a name and rich and extensive [del]fanwank[/del] backstory, but I don’t have the energy to search for it. He’s a dick anyway - he tells Gold Leader to stay on target, but when Gold Leader gets killed, he bugs out of the trench instead of trying to complete the mission.
