Basically, everything at REI is perpetually on sale for you. As far as you are concerned, 10% is raining from the sky.
When you buy something using your member number, REI gives you 10% of the price back in an Annual Dividend in March. So essentially, they give 10% of your own money back to you.
Though you won't receive a refund on certain products (like those already discounted), you will essentially have a coupon to the store in general, which makes spending on best-selling products that never hit the clearance rack feel a little bit better.
While the membership is $20, it's also for a lifetime, so it'll pay for itself over time.
And even if you don't have much faith in yourself ever dropping enough money to physically see that $20 fee in your pocket again, family members are able to use your membership as well, making it easier to rack up purchases and your annual return.