Not every bank has enough highly skilled developers and people to develop it's own software for Middle Office. 

It's difficult to keep up to date with all the changes, and regulatory requirements for a bank. The outsourcing software ensures that all rules are up to date.

It's cheaper to buy an outsourcing software, instead of developing software in-house

It's difficult to maintain a software technically for a bank, when traders or users request modifications all the time. 

Middle Office software collects information from different departments and allows managers to impact decisions.