Usually, we say this after completing some action for another person. For example, we might say it to a child after we zip up her jacket, a waitress might say it to a customer when she puts the plate of food on the table.
Some people use this expression in other ways...for example, to express support or acknowledgement when another person has just made a statement.
"I've finally decided to start college in the fall."
"There you go! I'm so glad you figured out your direction!"
Also, people say, "There you go again!" when another person is repeating a behavior that has been noticed in the past. Usually, this is said in a somewhat insulting way. President Ronald Reagan is famous for saying this in a presidential debate when his opponent started to "rant" (in Reagan's opinion) about something. Instead of giving a rant of his own, in return, Reagan just shook his head and said, "There you go again." It was a big "put-down" (insult).