In a phrase: to save money.
Medium and large sized organizations often have full time technical support staff on site, and they can afford to employ that full time staff. Individuals and small businesses usually don’t have the budget to do so.
By hiring an on-call tech support service like Personal PC, you can get the same level of service that you get from the box-stores or a full time tech support professional at a faction of the cost. You only pay for the service you need.