As Apple puts it:
An Apple ID is the login you use for just about everything you do with
Apple, including using iCloud to store your content, downloading apps
from the App Store, and buying songs, movies, and TV shows from the
Bear in mind that an Apple ID goes beyond iCloud and the different Apple Stores. Apple requires it if you want to interact with many of its services like (partial listing from http://support.apple.com/kb/HT5616):
Apple Online Support (apple.com/support)
Concierge (for appointments at Genius Bar [at an Apple Retail Store])
FaceTime [Apple's own video-chatting and VoIP technology, similar to Skype]
Find My iPhone
It's clear that Apple wants you to create an Apple ID, preferably with credit card information.
That said, an Apple ID is not required for logging in on your Mac - and if you create one, you can do so without entering any credit card information, see http://support.apple.com/kb/ht2534.
I'd recommend that you skip the step now and create it later when you notice that you need one (and you will :-).
(Take a look at Frequently asked questions about Apple ID for more information.)