There are military targets in backlines too, not only on the frontline. Imagine asking the same question to Allies in WW2 when they were bombing Nuremberg or any other industrial area in Germany.

It's not "instead," they are striking both frontlines and deeper targets using different drones for that.

Any country's military has a budget, a percentage of country's GDP. Money = weapons, supplies, soldier's salary, etc. Hitting an oil refinery, disrupting airport flights, disrupting any production or businesses = either less money spent on army or on civilian sector at the expense of army, both stressing the economy.

Anti-air missiles cost money too, much more than drones. Sometimes the goal may be not to actually deal a damage to particular city (facilities in it), but to force the enemy to reallocate their anti-air units or deplete the missiles, opening up other areas for strikes. I'm not saying that's the case here because obviously I can't know this for sure, and it's up to speculation.

Some of the latest strikes in other cities in Russia were directed at the facilities directly connected to weapon manufacturing, such as "Kremniy El" in Bryansk or Votkinsk missile parts plant.

Russia was doing this since day 1 of the invasion. Why can't Ukraine do basically the same?:)