In an example future world, one can use some form of FTL to instantaneously transport any ship from one point to another point (within the same galaxy, to keep things simple). The device required to do this is not excessively complex or large for an ordinary spaceship and does not require any sort of special positioning or calculation. These ships can move instantaneously from any position surrounded by hard vacuum to any other position containing hard vacuum (technically one can leave from in atmosphere, but that has nasty results for the surrounding area when suddenly the atmosphere contains a chunk of hard vacuum). However, another device that can block the entrance of FTL ships within a certain radius of it exists and can be reasonably used to block FTL travel within the orbital sphere of a habitable planet, within weapons range of a fleet of warships, or even within the inner system of highly developed systems. However, these devices do not prevent the egress of FTL capable ships.
This means that any ship can instantly retreat from a battle to any safe system, or even to some random interstellar location. However, this also means that when enemy ships jump to just outside the FTL restriction range of a fleet, the fleet can just instantly jump to somewhere else, which makes it virtually impossible to force a battle. Unfortunately, this means that two fleets will only fight if both think they can win the battle by a wide margin, which means battles will be quite rare, and this lack of battles makes military SF rather difficult to write. Therefore, we need a way to ensure that battles will still happen with some frequency. So, how do we ensure that battles are still frequent during wars when ships can escape with impunity?