My understanding of why they target refineries etc. is to reduce income for the Russian state. Every $1B burned is missed income that translates directly into less funding for the military and pressure on the state for all other things needs to pay for. It’s something that takes months or even more than a year to become visible because finances and budgets move slowly, but it will become noticable at the frontline and behind, e.g. due to degraded, irreplacable air defenses.