Is it for more rigidity, more motor placement options, or is it for something else?
Both. a 1X3X1 channel would be the most popular if they made it.
I second the 1x3 c c-channel. The 5X c-channel also allows for more gear mounting options if people chose to go that route.
Just as SBD said, more motor placement options and stronger…
Additionally, many teams use a horseshoe style base to make room for their lift and intake, and using any type of 1x2 metal for this can cause warping(first-hand experience).
Extra weight on the base also helps with COG issues and alleviates tipping, which are generally desirable unless you’re trying to high-hang.