Why do I need a credit card if I am only downloading free downloads.

First off, no one needs a credit card to create an Apple ID. You need an Apple ID to post here in the Apple Support Communities, so we know that you already have a valid and operating Apple ID. Were you required to supply a credit card to post this topic?

What you are being asked for is a credit card to use your Apple ID to open an account to use with the Mac App Store and iTunes Store. Provinding a credit card is a form of verifying your identity as the one who owns and will be using the account and the person to whom paid and free content is being licensed. If you don't see the None option in the approved payment methods and you refuse to provide the requisite credit card, your other alternative is to redeem a small iTunes gift card.

Late 2012 Mac mini, OS X El Capitan 10.11.2; Apple Watch, 38 mm silver AL, Watch OS 2.1; iPad Air 2 & iPhone 6+, iOS 9.2

Creating an iTunes Store, App Store, iBooks Store, and Mac App Store account without a credit card

Read the instructions very carefully. You will probably need to create a new account for this, then use it to download free apps, only.

Again, you aren't creating an Apple ID, you already have an Apple ID or you and I wouldn't be communicating right now.

You are creating an iTunes Store/Mac App Store account.

But you know what, go follow Kappy's link and open an account without a credit card and make a second Apple ID in the process. Then abandon your current Apple ID. It shouldn't matter, you haven't bought anything with your current ID.