The spread is the margin built into the exchange rate. If the dollar is at a certain value in the market and the conversion on your withdrawal uses a worse rate, that difference is the spread, and it's revenue for whoever did the converting.
It matters because it doesn't show up in the fee table. A platform can advertise a lower transaction percentage than its competitor and still deliver less money to your account, because the difference was charged on the exchange rate.
The honest comparison between platforms isn't fee against fee. It's how much lands in your account at the end of an equivalent sale, already converted.