Your arteries hold blood, but they also get clogged up by fat. If there is a clog, then the heart doesn’t circulate blood as well, which is bad for your body and also bad for your heart.
When your heart beats normally, the fat can form into clogs. When your heart beats faster it also beats harder, which dislodges some fat. This clears up your arteries, so when it goes back to normal pace the arteries are more clear.
ALSO, exercise makes your blood vessels wider. This means that even if you don’t have fat in your arteries, exercise still helps your blood flow better.
ALSO, exercise causes your body to release chemicals that regenerate muscle (oxytocin). This means that if your heart has ever suffered any damage, some exercise will help it to recover.
ALSO, exercise makes your heart grow stronger (and bigger). That means you end up with more heart muscle which means pumping blood becomes easier.
ALSO, exercise is good for your body in general. Everything in your body works better with exercise, and if your other organs are working better then they will keep the heart healthier too.