Some traders choose to invest in altcoins because they believe in the vision of the project or the use case of the coin. Others may look at hype surrounding the project or major upcoming milestones.
The appeal of investing in an altcoin is, generally, the high potential for profit. Altcoins have a considerably lower market cap than Bitcoin and so there can potentially be large gains.
With market cap, the price per coin is not the value of the coin. The value is reflected in the market cap itself. Low market cap coins generally have more room for growth than large caps, but they can be more of a risk.