It’s a great question, and one I’ve been a little confused by as well. The conclusion I’ve drawn is, it’s expensive to leave cache files for two reasons:
- If your site (or sites, in the case of a multisite installation) has/have a lot of pages, you can easily end up with thousands of cache files, which makes finding the right cache file a little slower.
- It’s easy for updates to be overlooked (e.g. change to a theme file, plugin, widget, etc) and so if you had no garbage collection and required yourself to remember to manually clear the cache, you leave a lot of room for human error.
As the plugin documents, there’s no “right value” to put in there. If you rarely make changes to your site, and you don’t have thousands of pages, I see no harm in making this value a day or two. Then, at worst, a page is stale for that length of time.
Again, I’m not saying this is the answer, I’m saying this is what I think is the answer. This question is over 6 months old, so if you’ve found a better answer since asking this, do share!