Suppose we want to run a complex reporting query on tables in Sales database (running MySQL) and Finance database (running PostgreSQL). What is the good option without duplicating data from one system to another?
Granted, it's not quite as quick as this, and not suited for "one-off" reporting, but it's a lot more flexible. Put things in a nice star schema and you have a good way to do analytics on your data.
Having a process that copies data into a reporting system solves the problem that otherwise it would be brittle (changes to either system might break things) and performance (any reporting queries affect realtime ones). Also, as it's decoupled, pulling in data from a 3rd system, e.g. CRM is possible.