The key is ... you have to level the city(s) first ... City(s) supply strats ... take out the City(s) and the strats won't be resupplied.
Then when you level the grunt strat ... any troops that are disabled on airfields for that country are rendered useless for up to 2 hours.
The whole concept of 'strat' is to initiate attacks on vital resources to deny the enemy the potential to fight back.
In other words, a regular scale of attack on vital resources should be more
efficient than pushing every enemy field encountered during advance with humongous numbers of planes, and gaining local airsuperiority with a 1:1 attrition scale.
The reality is, it isn't.
Going after the strats does something to the game, except the overall efficiency of hitting a strat target is so low that people would rather use all that manpower in hitting individual airfields on a tactical scale.
It has three tiers to make it effective ...
City -> Strat -> Airfield strat.
If they are leveled in that order ... you have made a difference and if you keep the City(s) leveled, it will continue to make a difference ... and then you will get noticed and you would probably see all sorts of "bomber" killers in the air.
The 'tiers', and the effective downtime, is the main problem here.
A bomber group successfully makes it deep inside an enemy territory to level the enemy city.
By the time they actually return alive, and then up another bomb run to go hit the strats this time, the city is starting to up again.
By the time that bomber group hits the strats, returns, and then gets ready to hit individual fields, both the city and the strats are back up.
So, this time, you actually make three bomber groups and go hit the city, the individual strat, and the field all at the same time... and now, since your mission has pulled out so many people from flying active fighter duty, that your whole front is pushed back.
Just leveling a strat (such as Grunts), for all intents and purposes, does nothing ... except help your "rank" and that is the only reason why people spend as much time as they do leveling those places. Their intent is not to hurt the enemy ... it's to bolster their l337 rank.
No arguments there.