The OpenSSL project offers the possibility to source out cryptographic functionality to plugin modules called engines. Usually there is one of two reasons for doing this, performance and security.
The performance reason is rather obvious, specialized hardware can do cryptography much faster than a general purpose computer.
The reason for using the opensc-engine typically is a security reason. If you are storing your private keys on a harddisk there is a lot of things an administrator (or a virus with root privileges) can do to steal your key. If the key is on a smart card there is usually no way to export the private key, so if you pull the card from the reader noone can use your keys. And if you use a certified and sealed reader device you can even be reasonably sure that noone can steal your PIN.
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