Why destroying a system can defeat the purpose of taking it

Why not immediately destroy the defenders with the much larger Imperial fleet?

The system's economic and political value gives the attacker a reason to obtain compliance while preserving it.

A Mid Rim system's wealth and political influence give the Empire reasons to preserve what it intends to acquire. Overwhelming strength could secure surrender without the losses and destruction of an assault. The deployment debate continues over whether to arrive with most of the fleet or build up through successive groups. Holding some ships outside the system provides a reserve for reinforcement or pursuit. A smaller initial group risks becoming a target for a defending fleet before the rest arrives.