The whole point of a L.L.C. is to limit your liability. So, if someone sues your business, you are only liable for assets of that business.
If you have business interests that are completely different, it makes sense to separate those into separate LLCs so that one lawsuit doesn't kill all of your business interests.
And it allows you to do interesting tax strategies if you create a "management company" that manages all of your separate LLCs.