It is incredibly useful to talk about a loop's trip count or the values an induction variable can have.
It turns out that taking advantage of signed overflow lets us do that in more places, a boon to all the programs which don't dynamically execute signed overflow.
The tradeoff, however, is that the programing model gets way more complicated. I have talked to the engineer who introduced this optimizing power to our compiler and he sorta regrets it.
IMO, we need better, higher level ways of talking about a loop's iteration space. Today, in C or C++, we are forced to represent the iteration space of a loop using the language's algebra. This is severely limiting when each operation on a signed type might introduce UB. With a higher level mechanism, we can separate the behavior that we want for loops from the behavior we want for everything else in a nice, clean way.