Most people are capable of handling the day to day operations of a couple of rental properties. And many feel they would rather keep the manager's fee in their pocket as cash flow.
I was of that mindset starting out, first with my spreadsheets then a free online management software; first with doing my own maintenance, then hiring out. I feel like it is the natural progression of a real estate investing career.
What most (including me starting out) fail to recognize is you may not be assigning a proper value to your time.
Your time may be better spent working to increase your income to buy more properties or in finding new properties through networking or creative means.
Plus you don't want to get to the point at which the stress of operating your rentals is holding you back when looking to grow your portfolio. My portfolio rapidly expanded and quickly overwhelmed my rudimentary systems, leading to stress and a certain amount of burnout.
I then hired a manager basically out of desperation, and it did not go well. In fact, it was such a bad experience that I ended up starting my own management company with a couple of partners.
Again, all of this is from my personal experience - others may see it differently.
Another benefit of a mgr is the fact that they should be able to help you as you evaluate properties to purchase (rent comps, repair estimates, etc.). A manager probably won't want to do this a bunch of times for someone who is not a client, but if you have a relationship established and the manager knows you are serious as a buyer, he or she will likely be willing to help you with your numbers. I know I help clients all the time with rent comps, etc. for properties they are considering buying.
TL;DR: Hire the right manager early and definitely not after you are already overwhelmed.