The Ex Parte Reexamination is a fundamental tool for fighting against patent trolls:
https://en.wikipedia.org/wiki/Reexamination
They can also be used by big companies to steal IP from small inventors. However, this is not why the backers of this bill are trying to limit them.
When a troll buys up a patent from the early 2000's, they hope to stretch its claims, with the help of a patent-friendly judge, to cover some modern technology. Naturally, it's the FAANG and other big companies they really want, but first they build up a war chest by settling with smaller fish.
Filing an IPR is a cheaper way than going to trial for challenging these bogus patents, and believe me, nearly all software patents are bogus. I busted lots of them, including this:
https://www.zdnet.com/article/microsoft-patent-may-block-goo...
If you go to trial, it's some unsophisticated jurors who decide if the patent is valid. For a reexam, it's PTO people, who at least know what the law is.
So that's why trolls want to get rid of reexams: to force companies to negotiate with them.