Breaking the linear mold
GTA 4 dipped its toes into branching choices by letting you pick who lived or died a couple of times. GTA 5 went a bit further with multiple heist approaches. But after playing Red Dead Redemption 2, it is pretty obvious Rockstar wants to make their open worlds feel more like RPGs.
Building your perfect heist crew
The most obvious way side missions will impact the main story is through crew recruitment. If Jason and Lucia are planning massive robberies across Vice City, they can't do it alone. They need drivers and hackers.
Imagine ignoring all the side quests on the map. When the time comes for the big finale, you might be stuck hiring amateur crew members who end up crashing the getaway car and forcing you into a massive shootout. But if you take the time to do optional missions for a skilled getaway driver, they will owe you a favor and make the main mission a total breeze.
Making friends in low places
Leonida is huge, and the leaks hint at a bunch of different factions running around the swamps and the city streets. If you do some optional work for a local gang leader, they might grant you safe passage through their territory when the cops are chasing you during a story mission.
On the flip side, if you skip their missions or actively mess with their operations in free roam, they might ambush you when you are in the middle of a totally unrelated main quest. It makes the world feel alive and makes every side quest feel actually worth doing.