But isnull has some particular capabilities that are interesting, despite its limitations: only two arguments, specific to SQL Server, and uh… well, we can’t always get three reasons, as a wise man once said.
There is one thing that makes isnull interesting in certain scenarios. Let’s look at a couple.
Read the whole thing. You (hopefully) won’t be in this situation often, but if you do happen to find yourself in it for whatever reason (and you can’t re-write the query to something better), it’s good to keep this in mind.