Comment by pm215

If the team is that small and working on things that are that disparate, then it is also very vulnerable to one of those people leaving, at which point there's a whole part of the project that nobody on the team has a good understanding of.

Having somebody else devote enough time to being up to speed enough to do code review on an area is also an investment in resilience so the team isn't suddenly in huge difficulty if the lone expert in that area leaves. It's still a problem, but at least you have one other person who's been looking at the code and talking about it with the now-departed expert, instead of nobody.