It also means you can install a close to stock version of Android without all the vendor additions. In the early days of Android, HTC Sense & Samsung's equivalent were probably necessary since the stock Android experience was pretty poor. Nowadays these vendor UI layers are relatively pointless: mostly they just make vendor updates to phones more expensive to implement and therefore less likely to occur. Vendors still like them because they believe they give them a way to differentiate their phones from everybody else's, but the vast majority of users simply don't care.

If you have a recent phone, or a Google phone that gets timely updates, then you probably don't care that much about CM. If you can afford to buy a new phone every year then you probably don't care very much about CM. That still leaves a big chunk of the population who can benefit from CM: an 8 million strong installed base is pretty impressive for an Android distribution that usually requires you to void the warranty on your phone.