Sitecore Powershell extension is a useful and powerful tool for doing various activities in Content tree easily and quickly.
It can be used to migrate data from an external system. For example we can take a csv file as an input and create items in Sitecore content tree by writing a PowerShell script. The main advantage of this tool is that it saves time from manual work (if we have to create large number of items in Sitecore on a specific pattern). Also we have not to rely on backend code to do such things. we can create a script in our local Sitecore environment , properly test it in test environments and the run it in Production environments.
Another advantage of Powershell is to use it for reporting. For example we can easily provide reports using Powershell for a particular group of item with filters. For example number of templates in the system or number of controller renderings created.
One more advantage to quickly fix any data issues in the content tree which is not possible to do manually. I performed this multiple times to fix content issues in Production when accidently some data issues happen either due to wrong migration or to remove legacy data based based on some criteria.