The purpose of the combiner box is to be able to connect a number of arrays in parallel and then have one wire to the inverter.
As an example you could have 5 arrays of 2 panels which have an open circuit voltage of 104V, and placing 3 PV panels may exceed the Inverter O/C voltage specification.
So in total if each array is 11Amps as an example, then at MPPT would be approximately 84V at 55Amps.

A proper combiner box, should have a suitably selected overload disconnect device on each array to disconnect from the bus.
There are 2 reasons for this,
1. To be able disconnect and service one array if required.
2. To protect one array in the case that it fails for what ever reason, from the full power of the other arrays feeding into the faulty array, the over current should blow the fuse or trip the breaker to isolate the faulty array.
Using the above example, if the one array has a short circuited cell, which can happen due to over heating, then that specific array voltage would be lower than the other 4 arrays. This would cause the other 4 arrays to directly feed into the faulty array, causing 44Amps to flow into the faulty array at midday. This extended over current, would cause the faulty array to heat and might possibly cause a fire.