It is the clothing that the players wear.
all of them do
BARRY BONDS
New Era 59fifty
Mark Teixeira
Pablo Sandoval
Mankini's
Alex Rodriguez
Depends how on the weather, I suppose.
Sixty percent
NBA players have to wear a uniform to be recognized as to what team they belong at. It would be harder to distinguish if a team is wearing different jerseys. It is also a pride of a team to have their own uniform.
The baseball almanac website provides a list of MLB players by year they retired. They provide a comprehensive breakdown of every retired uniform number in the National League.