If you own a business and have a number of IT equipment that needs to be looked after and maintained then you may wish to outsource this to an IT specialist company. Unless you have a large workforce working on IT systems then you probably wont find it cost effective to hire your own IT person unless you have another role that they could fill as well.
If you only have a few computers computers, do not use servers and are quite confident with IT then you may be able to manage it yourself, but if you have multiple machines, need to be able to quickly share and access information between each other, ensure that a full back up is carried out and that any issues that arise will be sorted out promptly then you consider hiring in a company. Their fees often vary from a support package which will give you a certain number of hours each month for support and updates to a pay as you go type system where you only pay for the hours you have used that month. If you want to know what you are paying each month and suspect you may have the need for IT services often then the monthly package is probably the way to go.