One of my main reasons for preferring a flying suit is that IMHO it helps reduce the hazard of loose articles in the cockpit. It has loads of pockets with zips which are mainly in the right places and accessable even when wearing a harness.
This may be a residual habit from the RAF but loose coins, sunglasses case, pens etc. can be a real danger....
Another benefit is that it could reduce burn injuries in the event of an accident but I value its practical features more. It also keeps my civvies clean when doing a walk-round when oil, fuel, dust and dirt can spoil what you are wearing
. The only pain is having to empty all those pockets when its time to throw it in the wash.
MB