It's much simpler than it sounds, only 7 lines of code in the sample posted. The point is that most people don't want to share their private key or their users' private keys, justifiably so.
The alternative is building the full binary transaction on your end, which involves selecting inputs, generating scripts, worrying about fees, etc. Implementing that is a week's project. Using an API is 5 min.
The alternative is building the full binary transaction on your end, which involves selecting inputs, generating scripts, worrying about fees, etc. Implementing that is a week's project.
They have. But only if you're on node.js and you need to run bitcoind yourself. Which means maintaining it, backups, restart scripts, etc. Building the transaction using BitCore isn't really simpler either (see the sample https://github.com/bitpay/bitcore/blob/master/examples/Creat...).
So using BlockCypher's API to do that is much more portable (you have ECDSA signature libs in most PLs) and avoids all the hassles of additional infrastructure.
The alternative is building the full binary transaction on your end, which involves selecting inputs, generating scripts, worrying about fees, etc. Implementing that is a week's project. Using an API is 5 min.