Why use 100E resistor in max232 ic in serial pins 13 and 14?
In general, we can't know a specific reason why someone has put them there, but we can guess. They might be there to help the chip ESD protection to cope with even larger surges than without them. They might be there to help to decrease EMI from conducting and radiating from wires. They might be there to slow down signal edges. They might provide some protection for overvoltage situations. They might be there just because this is copied from somewhere else, so there is really no reason.