Qlik Sense desktop is not 100% necessary to deploy Qlik Sense applications. All development and usage can occur using the Qlik Sense server, in which case only a browser (even from mobile) is needed.
Where Qlik Sense Desktop is useful is when users need to quickly access data available from their workstations that is unavailable from the server (due to process or controls etc...) , like their own spreadsheet.
It can also help with preliminary explorations of Qlik Sense since you do not need to contact qlik for a trial license to use desktop.
Currently Qlik Sense desktop is also needed if you want to develop or refresh an app that is running in the 'Qlik Cloud'