In practice, bringing all firepower to bear was extremely difficult. A large fleet has things stopping it from marshaling all firepower. Logistical concerns can force it to spread out. The ships may simply take up too much area to be controlled and massed effectively. Ships deploying long distances lose combat power from crew fatigue, maintenance needs, and hull fouling. The enemy may refuse to give battle. Torpedo boats may disrupt the line of battle. A continental power can achieve success with its army, even if it loses the naval war. Life is complicated.
In Honor Harrington, none of that shit is true. Enemy nations aren't a coastline to be patrolled. They're a point target. They aren't something to be influenced, but defeated in a day. If you pull your ships into orbit of an enemy world, they have to surrender, or you're given broad authority to blow the bejesus out of them.
Fleets are basically point targets, too. The physical dimensions of the formation are irrelevant because weapon ranges are stupid long. That means they work with N^2. Their endurance is also stupid long, you can support fleets of any size from even a simple basing facility. (See: Filareta's fleet basing from some random no-name system) In real life, ships can spend a third of their time, tops, on station, but in HH, having a navy of 100 ships-of-the-wall means 100 are ready for battle when the enemy arrives. Somehow.
However, those ships can't reliably mass to meet the enemy because detecting them far away is basically impossible. You have no idea they're coming until they arrive. Therefore, keeping your fleet split and then assembling when needed, is a shit idea. It's a far cry from real life, where you often need to stay dispersed because basing facilities can only support so many ships at once. Instead, you both can and must stay massed until the enemy's fleet, which must also stay massed, is dealt with, and whoever wins that battle wins the war.