Cheaper accommodation. If you don’t have a membership card, you will get a temporary one. They add a sticker each time you stay at a HI hostel, and after a certain number of stickers, it counts as a membership card and you can pay the same price as members. But it usually works out cheaper to get a membership card in advance, if you intend to stay at HI hostels often enough to get enough stickers.

Sometimes you also get other benefits, such as a further reduction (or even free stay) at hostels in your own country.

In some countries, they may give the same price to members and non-members (e.g. Tunisia), or give a reduction to members rather than non-members having to pay more (e.g. Ukraine). But the above is the official rule which is usually valid in western and central Europe.