Erasing means that at that point you have nothing on the hard drive. "Restoring the OS" is unclear, but in the context (seeing a quicker computer) probably means installing the OS from original discs or from the Cloud. At that point you'll have none of your apps or files. As you restore apps and files from a backup, things get closer and closer to how they are today.

In short, I think that the advice you got on another forum is from someone who had a problem that they did not know how to fix, and so used the nuke and pave approach. This may work sometimes, but it is terrible advice for most people. It involves tons and tons of work and only fixes things indirectly.

And considering that it's frequently the case that audio gear used with computers needs specialized drivers, it's especially bad general advice for people using their computers for recording.

Trevor