You usually only get that option, if multiple wires in parallel are a likely possibility for the amps of the main breaker. And the likely capacity, is only going to be slightly greater than what you typically need for that main breaker.
For instance, you might have a 400A breaker that has "snowman" lugs on it, rated for either 1 wire up to 750 kcmil, or rated for 2 wires up to 250 kcmil. You would not have enough capacity to connect 400A to the breaker, and then daisy-chain another 400A to an identical panelboard, since you couldn't fit 2x 600 kcmil in this particular example. You could connect the 400A to this breaker via 600 kcmil, and then make a tap to an adjacent 200A panelboard via 3/0.