• It helps one compare different cryptocurrencies fairly (knowing which is better)
• It helps to understand how big or established a project is .
• It helps estimate risk and volatility
Let us also be guided so we don't make mistake. It is true market cap matters a lot, in this case it helps in comparing the quantity of cryptocurrency not, the quality. Because a project has a large market cap does not mean the project is to be trusted, has good tokenomics, and vision. We cant use it to judge which crypto is better. It only compares liquidity, adoption and visibility, which is vital too to know.
It is also good to know that some projects manipulate their market cap. Using airdrops and bounties to inflate the circulating supply and tamper the prices with low liquidity. We need to bare in mind that large market cap cryptos can also crash. A typical example is the "Luna coin" that crash still hunts me till today.