If you had this much buffer memory what are the reasons to have swap space as well?

Many programs do stuff once during startup that they never do again, sometimes creating redundant data objects that will never get accessed in the configuration its being run in. Eventually the kernel memory manager figures out that some pages are never used but it can't just delete them. If swap is enabled it can swap them to disk instead. It frees up that RAM for something more important. It's usually minor but every few MB helps.

I personally like having some swap as during low memory situations (which lemmy gets at least once a day on my small instance) everything slows down rather than getting culled by the oom killer. It's not a replacement for monitoring, but it does extend the timeframe to react to things.