In The Bunny Theory of Code, Nicholas Zakas underscores the reasons to emphasize only checking in code that you understand:
In my current role at Box, I’m famous for repeating the phrase, “no accidental standards.” We don’t accept that things are “the way” just because they pop up in a couple of places. When we see this happening, we stop, discuss it, and either codify it as “the way” or disallow it. We then update code appropriately before it gets too far. Through automation, code reviews, and code workshops, we are able to keep an eye on the code and make sure we’re all on the same page.