My question is why many traders to buy and sell use the spot trading (where there are buy, sell buttons and order book) with fees instead of using the "Convert" tool without fees?
It varies according to what you want. Do you want to trade or exchange cryptos?
In spot trading, you choose the buying or selling price at which you plan to execute the order at, and therefore, with a little technical analysis, you determine the buying and selling points, and from them you achieve profits by buying at a low price and selling at a fair price.
In spot trading, instead of exchanging cryptocurrencies at a specific price, you can choose the appropriate price with you and wait for some time until the order is executed in the order book, and thus you will get a better price.
It is true that Convert does not contain fees, but there are hidden fees, which is that the price of executing orders is chosen by exchange and it can profit from the price difference.
it is closer to
spot trading: exchange 1 BTC for 101 USD with 1$ fees
Convert: exchange 1 BTC for 100 USD