okay
go to a pool and swim to the very bottom of the 12ft deep end. sit there for about 5 seconds.
feel that pressure? yeah multiply that by like, 1000.
it's pretty hard to get stuff down there, much less a human being, without it being crushed into oblivion.
That's one of the reasons why we should go down there though.
We know for a fact that at extreme pressures there are organisms living there at this moment in time.
Researching them and studying how the survive these pressures could help us to design stronger materials capable of withstanding much greater pressures.
This is obviously all speculatory that we could synthesise things based on these studies, but it could lead to improvments in industry (industrial production with chemical reactions at intense pressures would be possible), improvments in Oceanic research/construction (consider sub-aqueous oil-drills and other such buildings) and even in space exploration itself.
If we ever wanted to go to high-pressure planets (I'm not thinking Jupiter here, by the way), then we'd need to find a way to design things that can withstand that pressure.