Versatility. It seems there's nothing I can't get help with through SEMrush. The main thing is SEO, but there a whole bunch of other stuff they can help with: brand monitoring, position tracking, backlink audit, SEO ideas., etc.
The tutorial videos are helpful, so take advantage of them. Otherwise the amount of info might be overwhelming at first.
Price is reasonable (we pay about $100/month) for how much SEMrush offers. It might be too much for small business to fork out (and might be that it's even unnecessary if you're a one or two person business), but I wholeheartedly recommend it for larger businesses.
Competitor tracking is very useful, especially because you can see competitor backlinks and use that information to your advantage.
For those who rely on social media a lot, there are also great tools for basic social media analytics.
Just make sure to connect all accounts (Google AdWords and Analytics, Webmaster Tools, social media channels, etc.) to get full use out of SEMrush.
Site audit seems to find new problems somewhat randomly at times. If I fix something, it suddenly finds completely new issues that for some reason were not there in the previous audit.
I would also like to get recommendation globally, not just by a geographical location (for example US or Europe). I'm not sure if there are some technological limitations for why this option is not available.
On-page optimization, and SEO improvement in general. Finds broken links, and other SEO issues easily. The backlink audit is also very helpful.