Is not necessary but it's highly associated as it is one of the few places where NSFW doesn't have much of an issue, just like Pixiv FANBOX (Despite both sites are getting more and more regulations due to the pressure by credit card companies and similar)
I still would recommend that if your work is SFW to use Ko-Fi instead, not only because the place already offers the same as Patreon but because your supporters won't need to have a credit card in order to support you, for as long as they have a Paypal or Stripe they can still support you.
Content under paywalls can be of any kind, the thing is that you have to make it as it is worthy for others to actually support you on a monthly basis, be it early access to webcomics or illustrations, discounts, commission priority, exclusive illustrations/galleries, etc that you won't post anywhere else.
I don't have a Patreon anymore, which was NSFW, and honestly it was more work for even less compensation than while doing commissions, so I would recommend to first have content done before hand and slowly putting it in there mostly to don't overwhelm yourself.
