If you're trying to rank in the SERPs, a landing page is necessary so that there's actually something to be indexed and rank. If your page is a redirect to a different page/domain, then the page won't index because the page that you're redirecting to will index instead.
For SMM, you could just post an affiliate link directly to the page you're promoting without needing to go through your own additional landing page.
It should be noted that if you know what you're doing, you can rank an affiliate redirect in the SERPs through cloaking/doorway pages, which will let you effectively have one central landing page for many queries. Which is like not having a landing page.
