i don't know why coinmarketcap has become popular for people as a go to when they want to check the price! it is the worst place you can check the price from. it gives you the average, the charts aren't accurate,...
Unfortunately, most cryptocurrencies, especially bitcoin, news outlets (save coindesk and CCN) use CoinMarketCap's information and charts as a reference for prices in their daily "bitcoin/altcoins is up/downbull/bear" articles. I admit, I often use CoinMarketCap to check prices as it has tools and markets lists that make navigating exchanges for trading opportunities easier.
well coinmarketcap.com is bad (in my opinion) but it is not useless. all i am saying is that when you want to look at a better chart specially for bitcoin, there are a lot of better options. for instance TradingView is the best place which also has altcoins, then my personal favorite is bitcoinwisdom which is faster and easier to load and finally bitcoincharts that i posted above.
CMC is very good when you want to find where each coin is listed or find their links such as ANN topic, website, source code,...