Not all places are ideal, sometimes the best spots are car-accessible. In fact, the vast majority of the game is either directly accessible with a car or very very close to where cars can get to.
Grenading enemy-held cover is how grenades work... in every video game and in reality since before the first World War. The only other time you throw anything directly at people are the 8balls and bricks, unless you went back in time to the Beta and one-shot people with concs. You could always throw just one grenade and push while they are running away from it. No one wants to eat those unless they know for a fact that they can eat you first. Even better if it made it there with some fuse time left, since even if they evade the blast they still have to wait for the grenade to explode before returning to their position, earning you some sweet seconds to run at 'em.
This is also something aimed at almost everyone else that doesn't have an HVR as well. From the STAR to the SHAW, any weapon that gains great accuracy from marksmanship mode can make great use of cover
Again, you don't have to do this, flanking really works in this game because you're in a city with block and blocks and blocks of open malls, alleyways, highway overpasses, wide open streets, warehouses, etc. This is how most HVR's get dabbed on. Not much you can do when some guy sticks an OCA in your spine and motorboats your soul into the respawn screen.
Also, the real meta of the game is about spawning, and getting your jerk friends and streamsnipers to block objectives with cars because collision scripting is really really hard somehow.