Altcoins are testnets for future altcoins or bitcoin (best case) or outright scams or even malware (worst case). None of it lasts very long and if you believe in any of these projects you're going to be a fool in the end. It's all smokes and mirrors.
Well the some of the downsides but also altcoins are tokens that can be mined just like btc. Bitcoin is the main reason this other coins started so they are all technically just some copycat of what a bitcoin is. There are other currencies that used in many different uses depends on how the creator made it. But i agree with earlier that most altcoins are just scams that sometimes talk advertise in nice ways just to gather investors and then suddenly stop projects.