A common scenario is you may have one person wanting to gift a membership to someone else, but just a month or 2. This is a technique you can use to add a temporary credit to someones account paid by another person.
Step 1.
Use charge a guest to take payment from the person paying
Step 2.
Create a discount good for x number of uses, equivalent to the amount payed in step 1. More info on how to do that here.
Step 3.
Apply that discount to the persons membership plan who its intended for. More info on how to apply discounts here.