Following are the reasons to recommend separate staging database:
1) The database log file does not contain the staging ETL operation resulting in a smaller sized log file
The size of the database & backup files will be smaller
2) If log/data files keep growing, we can see massive performance hits as page splits and non-sequential writes may occur
3) For high availability, we can avoid pushing stage data across the network
4) The recovery model for a separate staging database can be set to SIMPLE to avoid generating a huge log
5) The separate staging database can be easily moved to another instance or server for heavy data processing without affecting the reporting environment