...either you want more pistons in the caliper so you can put on a longer pad

or

you want a longer pad so you can put in more pistons.

Look at like an F1 caliper (12 pistons I think), or this Porsche race kit pic from AWE Tuning, which I think are 8 pistons. They are situated along a long pad (or pads). Either they do this becuase there is an advantage to having more pad against disc, or becuase there is an advantage to having more pistons...





Copy Paste URL for Verrukt's Toyota F1 picture of the rear caliper...

If it's pad against disc, then the only way to get even pressure against a long pad is to use more pistons.

If it's number of pistons, then I'd guess they'd do this to make the pistons smaller. Smaller pistons can be pushed further out to the edge of the disc maximising brake leverage.

I honestly don't know why it is, but that's my guess...