God will meet all our needs both spiritually and materially. The Philippian church has been generous to Paul and he is commending them for their generosity. Paul himself is content with whatever he has. Even if he is in need, it is God who strengthens him so that he knows how to be content in any situation (Phil 4:11).
But though he could have survived without the Philippians’ gift, and though he tells them that they don’t need to send him anything else, he is thankful for their sakes that they’ve been so generous. He knows that God will reward them for their generosity, and that makes him glad for them.