The only way I see evolution favoring synesthesia is if it helped with survival.
Chromestheisa is the association of sounds with colors. These colors can come in several different ways including height width and depth. This might be able to help visualize where a predator or where prey are or even who they are.
Let’s say you hear a twig break. With your ears you can figure out the direction, but a lot of the nuance of the sound is generally lost to the untrained ear. Was it a big creature? A small one? Do you know which species it was?
As the sound information went into a brain, the signals get interpreted and some visual representation appears in front of the creature ideally giving it a bit more information than just with listening. Some of the small things that might have been missed listening, such as leaves crackling might be represented in the shape.
If the surrounding creatures were very similar, for both prey and predator, this could easily help you not becoming a meal when trying to find a meal.