Hey snake, it depends a bit on which kind of 'tight' clothing it is that you have seen.
In weightlifting, especially also powerlifting there is a certain kind of training clothing gthat actually "helps" them to lift up to 50 kilo higher or even more.
In powerlifting so called "bench shirts" are in an own lifting class. There are equipment lifting classes as well as no equipment classes and each have their own competitions as the differences are quite big.
Whilst the equpment class, really just becomes a material war - as in: who has the most expensive high tech bench shirt. The non-equipment class is merely really based on the athletes strength on it's own.
So those are 2 different ways of lifting. And equipment lifters also traina nd lift a bit different of course more based towards their gear.
And then of course just the tight clothing, which became a bit more of a thing since last year for men too. As @Adawg38 already said, it hinders less in movements. Especially such as squats etc, and on a bench you don't need to worry that everyone can look into your shorts :P
Plus it keeps the siniews tight on the body and functions a bit like a ellbow / knee wrap would as well.
Some also swear on the termic effect of it. Keepinmg your muscles more warm and ready for load.
That#s the 2 differences i could point out for you. Hope that helped too =)