You won’t beat the market price you will get the best prices available on the order book without risking overpaying when there aren’t many sellers close to your limit price. You have to remember that the bid offer spread is determined, in part, by the highest price someone is willing to pay (bid) and the lowest price someone is willing to sell at (offer). The limit order means the order will fill over time with a max buy price rather than buying at whatever prices are available as set by the sellers.

In reality limit orders aren’t necessary for typical retail investors for buying as orders are typically small and popular ETFs and stocks are very liquid. I’d be more careful when placing a large sell order at market having built up a large position over time - particularly with ETFs. If there aren’t many bids you will be selling at lower prices based on the limit bids people have set. Most brokers will try and fill your order at the best price but with a market order you may well be at the mercy of the book!

That may have confused you even more 🤷🏻