CoinMarketCap is more for an overview of crypto assets, whereas Polar keeps track of all of your own holdings. So for example when you have crypto on exchanges and/or crypto wallets, you add them to Polar once, and it shows you an overview of your current portfolio, and it will be automatically updated if you trade on these exchanges or top up or withdraw your wallet. You can also keep track of transactions and soon check how much profit/loss you made with your trades.