Here’s a practical example of when more CPU cores can really help, assuming it’s of value to be able to use the machine for other purposes while backing up.
But the idea applies in general to any situation in which there is work to be done while some background activity is in progress.
As shown below, on a 4-CPU-core Mac, nearly all the CPU cores are utilized (400%). On the 8-core Mac Pro this is no issue at all; 4 CPU cores remain available for other work. On a 6-core machine it will also be quite reasonable.
But on a 4-core machine (iMac, MacBook Pro, 4-core Mac Pro), nearly all the CPU power is being used; any kind of interactive use for other purposes is not going to go fast or smoothly.