The main reason to balance the load is to minimize voltage drop. Let's say you have two 100 amp loads on a 200 amp panel. If you have both loads on one side, you have 200 amps on one leg, 200 amps on neutral and zero amps on the other leg. Let's say you have 10 milliohms resistance on each of your service cables back to the transformer. In that case you would have a 2 volt drop on the hot current-carrying leg and a 2 volt drop on the neutral leg, leaving you with a total voltage drop of 4 volts.
Now let's see what happens if you split the load on both sides of the panel: Now you have 100 amps on each hot leg and zero amps on the neutral. This gives you a voltage drop of 1 volt on each leg and ZERO volts drop on the neutral. The total voltage drop on each leg is only ONE volt. By splitting the load and eliminating the neutral current, you reduced the voltage drop by 75 percent!
Of course, if John T chimes in, he'll probably add that he likes things to be symmetrical just out of general principles.