Firstly, if a child goes on a school trip without uniform then they could get lost in the crowd if it is very crowded. At least if they are in uniform they would be distinctive in a crowd and would stand out in a cowd but not with the school.
Secondly, if someone would have the latest designer clothes then they would feel left out and like they didn't belong.
Thirdly, everyone is equal so no one will feel left out
Finally, if people represent a school without a uniform policy then it would look silly as they would not not be in uniform thus not being able to win for the school.