Lets look at Managed Email Security. Email security has become a complicated business and most businesses would rather spend IT and money time running business systems like CRM and other customer and revenue focussed systems than trying to filter their email effectively.

A managed security service offers multiple layers of defense at a price that is not affordable if you had to acquire each filter for your own use, you get this at an affordable monthly per user fee.

Managed Service Providers also spend a lot of time improving their systems and ensuring they adapt to changing types of use cases and threats and ongoing customers requests for enhancements. Their rate of change and updates will be more frequent than solutions you acquire and use in house.  

Smart people use managed services because they're more effective, require no management, avoid large capital costs and give them a predictable monthly cost.