There's many advantages to going with the middle man rather than the ending company, USUALLY it comes down to service. You're treated as a person, and not just another dedicated server.

For us, we lease almost all our equipment these days, and from our end it makes sense as we don't have to worry about extra hardware on hand or having to play with things.

From the customer end (someone buying from us), there's value added services such as monitoring, backups, automated reboots, management and probably the most important part, HELP. Many people with dedicated servers need to know what the heck they're doing. How to use WHM or cPanel. How to configure firewalls or troubleshoot issues. Most data centers will charge you an arm and a leg ($165+/hour for a system admin), whereas going through a middle man, usually you can get these sorts of services cheaper.

For us, our biggest selling point is support. Most hosting companies don't offer "one on one" support in lay-man terms. Usually it's an engineer explaining something to someone who barely knows how to initiate a PING request