It's like they're playing follow the leader in the crypto playground. Bitcoin takes the lead, and others often tag along. Investing in different cryptos diversifies your portfolio, so you can enjoy the dance moves of each coin.
Investing in altcoins can be seen as portfolio diversification. But you need to know that it will not reduce your investment risk because, as you said, it all depends on bitcoin. If bitcoin falls then no altcoin can rise. But diversifying with altcoins that can maximize their returns grows unevenly. Bitcoin can go up 100%, but many altcoins can increase by several thousand percent.