It may be field specific, but I've also never heard of anyone running a manuscript through a plagiarism checker in chemistry.

It definitely exists. It's particularly useful for checking text in the background/previous research section of the paper, where it's easy to forget that you or a collaborator had copy-pasted segments from another paper's background/previous research section as notes to synthesize, and then genuinely forgetting because two months later, it certainly looks like something you wrote.

In contrast, for the main body of a paper in chemistry, it's probably not particularly useful.