You ground air to play air.
I grind my american tech tree to fly cool american carrier based aircraft. I have rank 6 american planes
I grind tanks to play tanks.
I ground my british ground to play look looking early cold war tanks like centurions.
I ground my german ground to get tigers and panthers.
I ground soviet ground and air for easy mode ground rb battlepass challenges and tasks (KV-1 zis-5 is a bit broken in downtiers, yak-9 speaks for itself.) Also t-34s look cool.
Air-wise, grinding british air for me is a waste as american air is bluefor, british air is bluefor - so I’ll be on the same side of the matchmaker in both cases and the reason I want a second air tree is ability to join the lower pop side. Grinding german and russian air is handy as they’re redfor, but neither tree has cool carrier based aircraft (russia gets 2 vtols at top tier.) Japan air interests me (ww2 is almost all naval!), but japan tanks don’t.
And I also want my RP gains to go towards a tank, not a plane. A good tiger game is ~5K RP surviving 20 minutes, getting 10 kills and the like. I spawn a plane to counter the corsair spam and I get 0 RP towards tiger IIs.
Ground RB air combat is the worst of ARB and sim too: It’s small, it’s cramped, it’s claustrophobic full of third partying and planes phasing into reality… and it has horrible rewards. If I want rewards, I got ARB. If I want fun, I got ASB.
Edit:
Clarifying on cramped & claustrophobic:
You see enemy plane. You dive on them until they’re within reasonable gun range. They’re diving away and maintaining speed. Your difference is about 100 km/h with advantage due to cutting off their flight path.
2-3 kilometres later: you’re over their airfield and got turned yellow with an oil leak by their airfield AAA.
Inverse is true: you extend away after making a gunpass and… your enemy cannot aggress you back as you’re right back over your AAA while trying to create a 2-3 km distance to re-engage.