In the gen 3 games, if I'm walking in a route that pokemon are encountered in, I can occasionally hear a quiet cry of a pokemon encountered in the route. Why?
One of my favorite touches from Gen 3 tbh. I'm guessing they implemented it to make the routes feel more atmospheric. Worked for me, at least.
They did this with Sun and Moon, too, I believe.
Oh, OK. I thought it had an influence on what pokemon you encounter! Maybe even a way to encounter a shiny easier. Like you listen for the cry, then walk in the grass, encountering that pokemon and you eventually get a shiny if you repeatedly encounter the same species.
I did hear a Rattata cry twice and encountered them (though the third, I encountered a Pidgey instead)
It's for auditory aesthetics. I believe that cries can also be heard in the DPPT, HGSS, and XY. I'm pretty positive that BW and BW2 did as well but the cries were more subtle.
Pokemon is a very fun anime, I like watching.