Hello automation engineer;

5V is used to supply the logic circuits in the CPU and modules, while 24 V powers the modules themselves.

This is described in many basic manuals of the S7-400, for example this one:

S7-400 and M7-400 Programmable Controllers Hardware and Installation

Electrical Supply

The modules inserted in the rack are supplied with the required operating voltages (5 V for logic, 24 V for interfaces) via the backplane bus and base connector, by the power supply module fitted in the slot on the extreme left in the rack.

Hope this helps,

Daniel Chartier