The membership is the whole reason they can offer the deals they do.
Costco revenue is about 2% membership fee and 98% sales of merchandise and services (most of which is merchandise, not services.)
Now, the membership fee is its main source of profit (because the merchandise sales are extremely low margin), but not its main source of revenue.
Costco derives the majority of their profits from the membership fee:
What it buys for me is, "not Walmart People". Totally worth the investment.
I know what you're thinking, but if a Costco membership is elitism, then fine you can call me elitist. Along with apparently 30% of the American population over the age of 18. We're the big bad 30-percenters, I guess.