The simplicity of use, it's free for small businesses with less than 100 users! You can see very detailed reports about your visitors' activity in real time without any additional software or plugins needed to install - this is something that I really like as an SEO consultant who helps clients find new customers through organic search results (SERP).
It would be nice if you could have some more advanced features but all things considered we don't need them since our needs are simple enough already so no complaints here at least yet :) We're solving two problems : 1) How do know what people look up when they visit my websites? 2) What content should i create next month/year etc.?