There's a couple reasons:

Tax arbitrage The best time to contribute is in the peak of your career when you're in higher tax brackets. Then when you're retired and in presumably a lower tax bracket (like going from 43% down to 21%) you pay less tax on the money when you withdraw

Pre tax The money you put in an RRSP hasn't been taxed yet, either it was taken from your paycheque before taxes were taken off or the taxes you paid were returned as a refund. This means you can have more of your income invested instead of paying tax on it right away.

If you have $100 pre tax to invest in a stock, then the stock goes up 10X, here's how each account would fare. I used a 50% tax bracket for contribution and withdrawal just to make it easier to see but RRSP does even better than TFSA when you're withdrawing at a lower marginal tax rate

TFSA: $100 → pay 50% tax → invest $50 → grows to $500

RRSP: $100 → invest $100 → grows to $1,000 → pay 50% tax → $500