dApp development tools are terrible; auth is too complicated (no-one is going to carry their keys around); dApp security is hard; contract immutability is not practical; build projects that were not possible before
substrate - Library Polkadot has been written on. A framework for creating cryptocurrencies and other decentralized systems. It guarantees cross-language support with WebAssembly, a light client, and off-chain workers, allowing for integration with other technologies.
Building a Crypto Company: e.g. you build a social network backed by some token. Early adopters using it get tokens for liking pictures, etc. so are incentivized to use it. Then if the platform matures, those tokens are worth more, and the early users are rewarded.