Trading exchange platform is the platform where people can exchange their cryptocurrencies to FIAT money or another cryptocurrency which ever they need. The most common pairings on these exchanges are Ethereum and Bitcoin. Some popular exchange platform are Bitfinex, Coinbase, BitMex, Poloniex etc.
Why not binance? I see Binance as the best of all exchanges and it seems that everyone here has been using it.
However, it is not necessary to opt for the best exchange. Just use the exchange there to have the altcoin you want to buy, and all the exchanges right now are legit