Are there any long term benefits to using single/two ply powerlifting suits? Or are they just used to allow more weight to be lifted during competitions? For belts, I've noticed better form and stronger core after wearing them - is there any similar benefit to wearing the suits?
The suits provide some compression, and more of a spring loading when you change from the eccentric to the concentric portion of the lift (bench press and squats). As a result, it will help you get out of the hole much quicker and more strongly. This in turn helps you lift heavier weights than you normally would.
The unexpected side to lifting with the suits is that most people who stall or fail a lift do it at the lockout. The reason is fairly simple: without the suit the most difficult part of the lift is getting the bar off the chest (bench) our out of the hole (squats) which means if you have the strength to do that, locking out is easy. With the suit, you have help getting out of the hole and the effect of the spring action becomes less as you reach lockout, so lockout strength now becomes the most difficult part. (Observation from David Tate's The Vault e-book at http://elitefts.com)
That said, David Tate does advocate doing most of your lifting without the suit, and only start incorporating it when you get closer to competition. The suits help people perform incredible feats like 800lb bench presses and over 1000lb squats.
They don't necessarily provide the same benefits as a belt does. A belt operates by helping you provide a more stable core, and with a more stable core you can lift heavier. The suits operate by providing more spring action to get out of the hole, so you simply have to provide the finishing strength.
The main reason is for competitions, but there are benefits. In fact, the great bodybuilder Ronnie Coleman wears a squat suit. A suit helps acclimate you to heavier weights.
Also benching in suit protects your shoulders. I'd say modern bench shirts are dangerous because they increase your bench to a weight you can barely control. But using a less aggressive bench shirt for your low rep is probably a good idea.