Sound bounces. In an untreated room, those reflections smear detail, blur imaging and let bass modes pile up into boom and suck-out. Acoustic treatment addresses this physically: absorption panels tame early reflections and high-frequency liveliness; bass traps in corners absorb the low frequencies that EQ struggles with; diffusion scatters sound to keep a room lively without harshness.
It is, dollar for dollar, the most under-appreciated upgrade in audio. Treating first reflection points and corners often transforms a system more than swapping any component — and it's why every serious dedicated cinema budgets for it from the start. Treat the room, then apply room correction to what remains.