Those who trade continuously on forex, commodity, stocks exchange they follow most currency indexes. Those who follow the index can understand that how important it is.
Index is very important for cryptocurrency trading too, so I shared the websites that I know. If you have more, share it here. https://coinmarketcap.com/currencies/bitcoin https://www.coindesk.com/price/bitcoin, https://www.tradingview.com/symbols/CRYPTOCAP-BTC.D