The main purpose of PHPExcel is to provide a library that allows reading, writing and manipulation of Spreadsheet files in a number of different formats.
Different people have different reasons for using it, whether to read spreadsheet data and load it into a database, or to take form or database data and write it to a spreadsheet format so that it can subsequently be accessed using Excel, Open/Libre Office, Gnumeric or other spreadsheet programs
PHPExcel provides functionality for manipulating spreadsheet data from within PHP scripts, but it is not a GUI tool... it doesn't provide a front-end interface: PHP is a server-side language, not a client side language. I have written a javascript GUI tool as an experiment, using Ajax to interact with server-side PHPExcel, just to prove it could be done; but it was highly inefficient (even on localhost) and certainly not usable over an internet connection (though it could probably done using websockets or similar, or a GTK application).
I'm not familiar with ZOHO spreadsheet, but if it can read Excel spreadsheets then it should be able to read the spreadsheet files that are written by PHPExcel.