They are useful for decentralized apps. The question is what is a decentralized app useful for.
The issue is that decentralization often comes at a cost. That cost is the time and amount of computing power required. The security of the blockchain relies on its size, so it can't be replicated on a smaller scale.
Most of the internet is about speed and latency. People are super impatient and don't even like half a second slowdown. Blockchain transactions can take minutes to a day. Computing is also power, energy, and heat constrained. Blockchains are heavy power, energy, and heat consumers.
What kills me about all of this is that, while the cryptographic tech is cool, the actual security is not that great. The decentralization helps against fraud but it doesn't help with trust and edge security.
In situations where you truly need unique signatures and you do not care about speed, like crypto kitties, or decentralized contracting (Again not sure why anyone would care about this), it can help.
But as a whole, most of computing cares about being faster, leaner, and better. Blockchain does not solve that. Look to cloud computing, novel forms of deep learning (capsule, complex, semisupervised, etc. etc.) 3d microchips, and even quantum computing if you want to invest in future computing tech.