To obtain personal information and login information

[robiulhasan]

A malicious purpose is to create a mirror site that makes people believe it is a real website, and then steal personal information and login information that users enter. These are called
"phishing sites," "fake sites," or "imitation sites."

To prevent server downtime due to concentrated access
One good purpose is to reduce the load on servers,
for example, in the case of disasters, when access to local government websites purchasing directors mailing lead is high.

SEO Issues
The problem with mirror sites in SEO is that one of them will not be displayed in search results . It is not possible for both to be displayed.
This was triggered by the Panda Update, an update to Google's search algorithm in 2011.
Since then, Google's search algorithm has lowered the ranking of sites with duplicate content, and now they are not even displayed in search results.

Be careful, because you will be at a disadvantage not only if you create it yourself, but also if it is created by a competitor.

Dealing with mirror sites