Hi again,
without proper decoupling a complex circuit won't work properly. Imagine one IC just switches demanding high current for a split of a second. Drawing high current will cause the supply voltage to surge and mess up another IC which wants to switch simultaneously.
It is always good practice to use an electrolytic capacitor on a board just where the supply voltage is provided. From the electrolytic VDD and Ground continue to individual ICs.
The IC power pins should have a decoupling capacitor as close as possible to their supply pins. Normally that cap is placed close to the VDD-pin of the IC.
The reason for that is simple: When the layout is almost finished all free space is used to enforce the ground connections. (In some cases occupying the entire top layer.)
Look at the sample circuit for better understanding. The circuit is a null-function circuit to show how to decouple.
Boncuk