Common sense.
I look at the suspended weaponry of an aircraft and can immediately venture a rough guess if it’s suitable for a certain role or not.
Having a basic knowledge of military aviation in general and the type in question helps as well.
Sure, but if you purchase a vehicle and don’t use its strength, but complain about its (obvious and easily recognizable) weaknesses, that’s on you.
Because armies always try to use their equipment for other things. The rocket armament was of limited use for close air support, but the J35 never was a real attacker.
Note also the designation J35, J standing for Jakt, indicating a fighter role. If it had an attack role, according to Swedish designation it would have an A for Atak in the designation, if it had also a reconnaissance role, an S for Spaning. Basically the equivalent of F, G and R in the British role designations (e.g. Phantom FGR.2).
See also the Viggen, which in WT comes as AJ37 (with ground attack as primary and fighter a secondary role), compared to the JA37 (with fighter as primary and attack as secondary role), and the AJS37 (like the AJ37, but with additional, in WT irrelevant reconnaissance role).
Or the Lansen, which comes in dedicated J32 fighter or A32 attacker variants. Also here the J32 has primarily an air to air loadout, with (even more limited than J35) air to ground secondary armament, and the A32 with a good, dedicated air to ground armament and no air to air weaponry.
If it comes to grinding btw, the A32A Röd Adam would have been a great and effective choice if you like ground attack / base killing as grinding method.
TL;DR: Use the aircraft for their intended role if you want to get the most out of them for rewards and grinding.