Using a monitor or TV to double or extend our MB screen is common, and mainly depend on your requirements, video editing, audio or image; games, or watch movies or videos while you work. or 12 "monitor you think small, in short, any excuse is good to get one.
There are no rules, only compatibility, perhaps, moreover, anyone can be useful. In particular I have to hand an LED TV that I use from time to time to see movies, or games that connect to the HDMI adapter, which is cheaper than getting a Thunderbolt monitor. VGA connector detract resolution, so it is only useful for projectors.
I let you down two league titles that may help you decide.
Regards.