Preventive maintenance is something that has to be done predictably, provably and regularly. Our proactive maintenance packages log our activities, and allow us to prove that we are patching, maintaining, monitoring and managing your systems before something breaks.
Help desk is something that is done as needed, and usually very strong end-users need a lot less help desk. Some customers are super stars at using and fixing their own computers. Helpdesk isn’t that useful for those customers.
Also, some customers have a lot of “general use” computers where the user doesn’t need helpdesk. For that reason it is not required for every computer.