This is a good way to ensure that your membership product will always be purchased with subscription properties. This allows you to avoid theme issues where recurring properties are removed from the product.
To force a membership product to be purchased as a subscription:
Open your Shopify admin
Click Products to load all of your products
Click on the product you wish to modify
Scroll down until you find the Subscriptions card
Check the option that says Only allow purchase of this product as a subscription
Save your product