https://www.polygon.com/2018/11/21/18104655/pokemon-lets-go-extra-pokemon-transfer-guide

One of the biggest differences between Pokémon: Let’s Go! and the previous main Pokémon games is the catching mechanic. Instead of chancing upon wild Pokémon encounters in the tall grass or the crevices of caves, you see all the Pokémon wandering around and can chose whether or not you want to encounter and catch them. And instead of battling them to the brink of death, you catch them à la Pokémon Go, either waving your controllers or pressing buttons in handheld mode.

While this means that Pokémon-full areas like the Rock Tunnel are more streamlined, it also means that there’s seemingly less incentive to go out your way to encounter Zubat after you’ve already added it to your Pokédex. So is there any point to catching more than one Rattata besides the fun catching? Actually, yes.