There are two main reasons for wearing straps.
1 When you always wear a strap you always have the instrument at the same hight wether you are sitting or standing and you get used to this position. You might not want to stand to play but some time you may not have any other option because of wet grass, or no chairs left in the session, or playing with others on stage when they are all standing.
2 It keeps strap makers lick Rick and myself employed.