"That is where I come in' is a phrase that you would use when telling someone a plan or a life story. It's not used very often when speaking to someone but you will probably see it used a lot in movies and books.
These are some of the ways it can be used:
Some characters in a movie are planning to steal a painting from a museum and the main character is telling the other characters his plan. He would say something like, "so you and Joey will distract the guards, whilst Gary breaks the alarm," Now, the main character needs to tell the other characters his part in the crime, so he might say ' and that is where I come in, I will hide the van around the back."
In this case you would use 'That is where I come in' to break up the sentence, because just listing what the characters will do would be boring; In movies lists need to be exciting, so the director adds this phrase to make it seem more interesting.
I hope this helped, if you need anymore help please feel free to ask :)