If you right-click the service, go to Properties, then the Dependencies tab, you can see what other services rely upon it. In this case, quite a few, basically anything to do with remote access, connection sharing, and fax. You can then look at the dependent services to see what functionality they have, in this case everything from dial-up, VPN, ICS, to faxing. If you don't need to log onto your ISP, don't use fax, and don't plan on using VPN or sharing a connection through that PC, you can probably disable it. I personally have it on manual as well as all the services that depend on it.