A well trained IT employee can command a hefty salary to manage your valuable IT system and offer in house tech support to your team these days. There are also the extra ongoing costs such as certification and education so that they can be up to date technologically speaking. When you add in the fact that they only work 48 weeks a year, it becomes clear that your network may not be always protected particularly at holiday times. This is even more the case when you have a sole IT person on your team.
When you outsource IT to a quality Managed Service Provider, you place the responsibility for your network management on a 3rd party. They will be part of a well-resourced team whose goal (like yours) is to ensure maximum up-time of your system and who will be there for you not only during regular working hours but at other times too. You will only be paying for the services provided, not for those extra certification and education costs. Monitoring of your system will be over the full 24 hour period, day in day out.