the problem of sending data that maps directly to your database tables

I was searching for the uses of DTO. Microsoft Docs mentioned one of the uses as the following:

The client receives data that maps directly to your database tables. However, that's not always a good idea. Sometimes you want to change the shape of the data that you send to client.

I would like to know what would be the problem if we send data that maps directly to database tables? will there be any security issues?