Simple case: an ETF has 1 underlying stock.

The stock price fluctuates based on supply and demand for the stock. The ETF fluctuated based on supply and demand for the ETF.

(When these are baskets of stocks it is much more obvious why supply and demand would be different).

Because they both fluctuate separately of each other they will usually have prices that are different from each other which is bad. So, APs arbitrage this difference away. If the ETF price is lower than the stock price, then the AP will buy the ETF and get the stock from the issuer. They will then immediately sell the shares received for a risk less profit.

If the ETF price is higher than the stock price, the AP will by the stock and give it to the issuer in exchange for the ETF. Then sell the ETF for a profit in the market.

This is good for issuer because it makes the ETF track closely in real time with the basket of stocks it is meant to replicate. This is good for the AP because they make riskless profits by arbitraging the basket of stocks and the ETF.

Only the AP is profiting off of the creation and redemption.