system
Given the following post, respond as if you're commenting on Reddit:
human
Why would evolution 'eventually remove' T-Rex's Arms?

I read an article about the arms of a T-Rex. It claimed that, perhaps given another 10 million years, the T-Rex would have no arms. Explain to me how evolution would get rid of 'useless' or perhaps vestigial traits. 

In my mind, evolution works against things that make an organism  less fit for it's environment - e.g. you're a moth in a very green forest with other green colored moths, but you happened to be red - that's going to be a problem for you. 

Why would having something 'useless' lead to it's eventual disappearance, as, in the case of the of the T-Rex, having tiny arms (again, vestigial?) is not an obvious evolutionary disadvantage.  

edit: Fixed a sentence I mistyped.
gpt
Tyrannosaurs' arms were small, but they were quite muscular as can be seen from muscle and tendon attachment marks, so it's likely they had some function to justify the muscles' continued development rather than atrophy as would be expected in muscles that were not normally used - so the arms were most likely functional and not vestigial. We don't know what that function was, though I've heard the hypothesis that they were necessary for (male) tyrannosaurs to be able to mate.

To answer your more general question about evolution reducing vestigial features - each physical feature comes with an energy cost for the creature to develop and maintain it, so an unused part would give an advantage when reduced because the creature would need marginally less food. Some features also might carry a risk of infection or injury, such as eyes in the case of cave animals, causing them to be reduced or eliminated as well.