Altcoins follow the trend for bitcoin, many of them are worthless and will only gain some value only when the market is in an excessive bull. Many of the top coins are not deriving value from use cases, only from occasional pumps and dumps
There are a lot of altcoins, which are increasing day by day. It is something above 5000. As you said most of them are useless and wasting-time. As you said every altcoin price depends on BTC price. When BTC starts growing up, altcoins follow the movement of BTC. For that reason, people make money from occasional pumps and dumps.