Based on the link someone put in a different comment about them suing Deno, at least in Oracle's case the answer is presumably "being able to sue people and get money from them".

Even if that weren't the case though, I think part of the problem is that even if the trademarks literally never brings any value, it also potentially costs them nothing to retain them (unless someone tries to get it invalidated, at which point there's some cost to trying to defend it). Arguably the cost to establish in the trademark in the first place is also low enough that companies at that scale don't have much incentive notto establish them in the first piece; they already have lawyers and trademarking things isn't really out of the ordinary for them, so the marginal cost of having them file one more isn't very high.

It's worth considering whether the point you make about there not being much of a realistic concern around someone else attempting to copy the name is something that would be obvious to non-developers. Sometimes what might be obvious to a developer might not be obvious to a lawyer, and at the end of the day, the legal team is probably in charge of deciding things like this at these companies, so in the absence of pressure from someone who understands this point enough influence to make it happen (like maybe a C-level exec), it might not matter that the concern is realistic if it's theoretically plausible.