Regarding the question in your title, I would say that price tracking is important to decide what price to buy or sell. Long term tracking is needed to understand a trend and then if your target price is certain points higher than what currently is ongoing and you see a bearish wall starting to build up then modification of your order is determined by that price tracking.
really interested in the question above because as I see, many traders don't use price trackers such as CMC and go directly to the exchanges where they prefer to trade. Or am I wrong?
CMC prices are late than those on the exchange. I would recommend the second one if you are day trading or starting to trade for that day. If you just want to glance across the market in the morning and wont be trading anything you can go with coinmarketcap to get a general idea of how the market was last night/day.