One of the most significant ways he can help Biden is by turning over all of this “volunteer” networks in place already in a bunch of states. These are the people who canvas neighborhoods, text & call potential voters, put signs on lawns, etc. For most of the other candidates, these are real volunteers. For Bloomberg though… they are paid staffers (I believe well over 2000). And now they will work for Biden. They are incredibly organized and well funded.
On top of that, Bloomberg will just dump more money into a SuperPAC to help Biden.
He will also face far less scrutiny as basically an ATM than he would as a presidential candidate. He was probably tired of releasing people from NDAs, and tired of answering questions about his Stop & Frisk policy.