Joining is free and entirely digital, and new members get 10% off a single purchase straight away. You earn a point for each SAR spent, and each 500 points brings a SAR 10 voucher you can redeem in H&M HOME or H&M, whether shopping online or in a store.
The membership page mentions further perks, some limited to the “Plus” level, among them a 30% birthday discount on a single item, extra savings during sales and free collection from a store. Collecting 1,500 points takes you to “Plus”.
Keep in mind that returning something bought with a voucher only refunds what you actually paid; the voucher itself is not reissued.
Source: H&M Saudi Arabia: membership information