In my opinion their strenght is in the fact that you actually...

In my opinion their strenght is in the fact that you actually own them. It's like having cash but in a digital form, no bank can block your account or stop a money transfer, if it is truly decentralised no one can stop you to move your money, low transaction fees and fast transfer. This is where cyptos like BTC have value imo.
For ETH and other platform smart contracts and other coins built on top of the platform create value, almost all ICO raised money in the form of ETH rising the price of it.
This is why cryptos have their followers and investors in my opinion.