Flexibility. It’s a concept that is almost completely lost with modern bikes. When I started racing as a junior in the mid 70s, and was carefully watching everything that the better Senior riders were doing, I saw a number of them who would move the wheel forward for a criterium or move the wheel back for a long road race. I think that there is a small difference in the way a bike handles as a result, especially with a relatively short wheel based racing frame. As others have mentioned, you can also “cheat” the capacity of a racing derailleur by moving the wheel back all the way for some extra cog clearance. I think that the greatest benefit that I have experienced myself, though, was the ability to take a spare wheel that may have been dished a little off…You can tweak the positioning to get the wheel centered in the chain stays even if it is a bit off….and it gets you back in the race ( or ride)…The same thing is useful if you break a spoke as sometimes you can mess around with the wheel position to get it in a spot where it doesn’t rub….