The chips on the hub require 5v or 3.3v power. If you are connecting the hub to a PC power supply through the 24 pin connector, it supplies 5v and 3.3v directly to the board from the power supply. If you use a 12v external supply, you need the regulator and capacitors to provide 5v and 3.3v to the chips.
The 4 port hub does not have any chips and simply passes 12v and data straight to the SSC. So, 5v and 3.3v are not needed.