* They will register all TLDs. A lot of more obscure ccTLDs (e.g. Norway, Hong Kong, Argentina) require domains are registered with a local company. MarkMonitor will set the local presence up for you and charge you a fixed fee. Some other registrars support some of these TLDs but none support all of them, so otherwise you're split between registrars. If (for whatever reason) MarkMonitor can't register a domain themselves, they'll go as far as getting a credit card out and using another registrar who can.
* They invoice you: you don't have to worry about expired credit cards or any other payment problems. You get one invoice every month for all your domains. (If you've ever had to deal with submitting expenses receipts for 80 domain renewals, this is definitely a bonus. Now consider that most huge companies have tens of thousands of domains.)
* They handle the domain "sunrise" applications for you (this is apparently a big brand protection deal if you're a big company), so you'll regularly get emails saying "do you want to register the .xxx domains for all your brands?".
* They do SSL certificates as well, and they automatically get renewed and invoiced.
* You get an actual friendly human account manager to talk to.
I can't remember what their actual pricing is. The price per domain is perhaps 2-10x more than other registrars. I'm not sure if they charge a recurring fee.