It can monitor smart contracts activity and interactions based on generated transactions and events.For example, It can be used a local blockchain explorer that runs locally on your server or machine ,or as an investigation tool that scrapes the blockchain in search for a specific query. This is done by sending requests to an Ethereum node via JSON RPC calls. There are public Ethereum nodes that can probably be used for this.

blockchain.info has a realtime ticker of Bitcoin transactions that their processing nodes have received through the blockchain but haven't yet been recorded as being confirmed as valid.