Great integration between forums and knowledge base. You can easily turn a great forum post into a KB article that helps the community forever. There are also a lot of great features like accepting an answer as a solution, giving out kudos, and things like badges.
Lithium lets you leverage your community to easily crowd source supplemental support and guides for your other users.
If you don't host your own forums/KB/wiki, your community will either do it for you in a place you can't control and some users will never find/trust, or your community will be too small to even matter.
Lithium is a great package that gives you all the tools you need to build a community. Rather than having long forum threads where a question is asked at the start and then maybe answered 20 of 30 pages in and never found again, you can mark that post as a solution and anyone else that finds the thread can jump right there. If someone posts a good guide, or even something close, it can be turned into a Knowledge Base article for everyone. If someone is just helpful, you can give them a bit of kudos to know they are appreciated.
You can also set who is able to do a lot of that stuff, whether just the general community or specific community managers or what.