Think of it this way. Breakers protect the wires. You don't want the wiring to burst into flames in the hidden recesses of the boat due to over current. So the breaker should prevent it. If you have a multitude of devices ultimately being fed to the breaker via one wire, then you should make sure that the wire is suitably-sized for ALL the devices to be on at the same time. This commonly occurs where lights are all connected to one breaker. Lights don't produce much current so it is norma for a 15 amp breaker to be suitable for a grouping of cabin lights with small wires - say 16 awg. But multiple lights going up the mast joined with a common ground may create an over-current situation when all are on and 16 awg may be too small due to voltage drop on long runs. 10, 12, or 14 awg may be the correct wire size depending on the boat. In this case, the breaker should be sized for the safe/anticipated current when all devices are on. On small boats, it's hard to image a circumstance where any breaker greater than 15 amp is necessary, so that is why these panels are built with 15 amp breakers - they are the most common. If you need a breaker with higher ampacity, you can simply exchange it because they are interchangeable.
Sometimes, or maybe even often, a single device such as a chartplotter will be on its own breaker. However, the chartplotter needs to be protected to 5 amps. It seems silly to put the chartplotter on a 15 amp breaker when a smaller breaker is suitable. But you already have the breaker in the panel. Besides, maybe you are going to put more electronics and radios on the same breaker. You still need to protect the chartplotter. So that is why you install the 5 amp fuse next to the chartplotter. The wiring is safe and so is the chartplotter.
