It saves you from having to host an email server yourself, which is frankly a huge pain, regardless of any guides one finds on the web.

Or, if you so prefer, it saves you from running Exchange, which gives you a more complex environment for more cash.

Furthermore, there are no ads in 'Google Apps for Work' or 'Google Apps for Education' [1].

[1] https://support.google.com/work/answer/6056650?hl=en