People often tell each other "Let's keep in touch", but often don't keep in touch with the people they say this to. And you sometimes use "Let's keep in touch" with people that you don't have any interest in talking to. So when you really want to keep in touch with someone, you have to say it in a stronger way.
Adding the word "definitely" to "Let's keep in touch" helps to show that you really mean it.
You may also need to add more emphasis afterward:
Let's definitely keep in touch. I'm serious.
Let's definitely keep in touch. I mean it, let's talk soon.