Use this expression when you want to talk about the good points of something:
The good thing about my job is that I get a lot of vacation time.
The good thing about living in a big city is that you can get everything you need within walking distance.
In grammatically correct, formal English, you should include "that" at the beginning of the clause:
The good thing about it is that it doesn't need to be watered that often.
In conversational spoken English, people often leave "that" out.