There are various reason to merge PST files following are some solid reasons to merge PST files:-
1. It is difficult to manage so many PST files in a single application
2. To fix Outlook errors caused by several PST
3. It takes more time to search for a file in Outlook if they are multiple files
If you want to combine multiple PST files into a single one then, you need a third-party tool that easily performs PST merge in few clicks. I would like to suggest try the Shoviv PST Merge tool which is easy to use and provide an attractive user-interface to merge PST's.

For more info visit here: http://www.shoviv.com/pst-merge.html