Why did Maniac Magee paint "101" outside his door?

Jeffrey paints the "101" on the door as a proclamation that the baseball shed has an address. It is now 101 Band Shell Blvd, home of Grayson and Jeffrey. This moment comes at the end of a memorable Thanksgiving holiday for both characters, possibly the best (as Jeffrey says). They have feasted and danced, and Jeffrey feels happy, secure, and at home. His giving the shed an address and painting the numbers on the door symbolizes those feelings and lends significance to this moment in his life.
It also marks the beginning of an important stage in Jeffrey's life, as he grows in this sense of home and constructed family. This emotional moment also adds to the sense of tragedy the reader feels when Grayson passes not long after, five days after he and Jeffrey celebrate Christmas together.