Using Role-O-Matic doesn't mean getting rid of s2Member, because Role-O-Matic doesn't protect pages. It is an add-on to WooCommerce.
Role-O-Matic enables you to sell roles, whether as one-offs or as subscriptions, but via WooCommerce. If you have s2Member activated, you can thus use Role-O-Matic to sell s2Member roles.
Obviously, s2Member already comes with the ability to sell roles, but using Role-O-Matic overcomes some actual or perceived limitations of s2Member in terms of that particular area of functionality.
First, if you use Role-O-Matic, you can then use any payment processor add-on that works with WooCommerce. So you aren't limited to using PayPal, Stripe, Authorize.net, or Clickbank as your payment processor (as you are with s2Member).
Second, some people want a wider range of information available to them for the purpose of analytics than s2Member provides. Using Role-O-Matic means you just need to activate an appropriate WooCommerce add-on to get what you want.
Third, some people say they have trouble getting s2Member to work correctly for payments (at least for PayPal). So they can use Role-O-Matic and make the connection to PayPal using WooCommerce instead.
Fourth, some people want to integrate a membership part of their site with an e-commerce part. Using Role-O-Matic means that this can be done easily via WooCommerce because you then don't need to use s2Member forms or buttons.
Five, some people want to sell other items together with a membership, or require an upload at the same time as selling a membership. WooCommerce makes both possible with the appropriate add-ons, and Role-O-Matic makes using WooCommerce possible.
Sixth, some people are concerned that the lack of updates to s2Member might mean that it ceases to be compatible with the APIs of the payment processors with which it is supposed to work. Using Role-O-Matic means that you use WooCommerce for that instead and, since WooCommerce is updated on a much more regular basis, there is no chance of such incompatibility.