Players can be superstitious about their uniform number. Some players succeeded early on with their number and want to keep that mojo going. Others may have chosen their number to honor a friend or family member.
When players change teams, they may want to retain their number, which means a new teammate may have to give up their number. For example, Andrew Benintendi was #16 with Boston, which probably means rookie Kelvin Gutierrez will have to give that number in deference to the veteran.
Carlos Santana wore #41 his entire career in Cleveland and Philadelphia. Danny Duffy has worn that number the past few seasons for the Royals.