If a membership is set up such that the automatic renewals are not turned on, the user might attempt to renew their subscription manually. However, with Paypal, the expiration date in SPMS is not updated to add an additional period to the expiration date.
For example, if the expiration date is currently August 1st and the user manually renews sometime in July, you would expect that the new expiration date is September 1st. However, with Paypal, the date remains August 1st.
Thus, for manual renewals where you expect the user to renew before the expiration date, you might want to consider using STRIPE instead of Paypal.