Copper wire has inductance which prevents current flow.
ICs do not consistently accept energy, they take it sporadically. For example, a SPI chip might be turned on to transmit a message. If you need to deliver current from a power supply 4 inches away, the electricity literally takes too much time to get there and the voltage line can dip. The close capacitor acts as a quick response energy reservoir to supply energy for those quick turn ons of the IC.