The main reason is repeated income. If you have many prospects who promote your product, then the monthly income also depends upon the number of prospects who are promoting products.
Especially what if your top affiliate promotes another which product converts better? What if your site loses rank because of the keywords it carries? What is your highest number of sales?
You can set your membership site in such a way that each member must go through content for 12 consecutive months. You can also know more about clickfunnels membership site at https://www.funnelsprice.com/.
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The way you can manage your membership site is to give members full access to all content once they join. Because you don't have to worry too much about what content you must have uploaded, you can now focus on getting more people to join your membership.
One thing to look after what you need to remember is that you need to make sure the content you provide is always updated.
You can also offer a fixed term membership for 6 months, let's say boot camp instead of a membership site, and get people to pay you only 6 months. The best thing about membership sites is that everything is automatic after you set it up.