Use membership plans when you want to treat a group of customers differently at checkout without creating a separate product. Typical uses:

• Member-only discounts: create a coupon (fixed amount, percentage, free delivery, etc.) and set Limit Per Membership Plan so only customers on that plan can redeem it.
• Tiered perks: create several plans (e.g. Silver / Gold) and give each its own coupon codes or auto-apply coupons.
• Corporate or partner programmes: assign a plan to employees of a partner company and target promotions at them.

Because a plan has no price or billing, you decide how customers earn it (paid offline, loyalty, partnership) and an admin assigns it on the customer profile.