Having said that, not all of them are reliable or can be considered fully functional as of now. Many of them are dead, defunct, scams, etc. which you can find on deadcoins.com.
On to your second question, it depends on the type of problem cryptos can solve. And there are problems aplenty in this world which I believe a cryptocurrency can solve.
If you look around on CMC (coinmarketcap) you will notice that there are different kinds of tokens and coins for different kinds of systems, industries, and markets - payment, governance, social media, storage, real estate, platforms, stablecoins, exchange utility tokens, prediction markets, insurance, trade financing, gaming, decentralized energy, moneylending, banking, supply chain - I mean you name it and we have a coin or token for that.
So we can have a crypto token for many different industries, processes, and markets. However, I would advise you to look at them with a pinch of salt for completeness, readiness, need, and without a doubt please DYOR!!