Imagine you get a brand-new employer-issued, Windows computer. It's fresh and pristine, and working great. It's got Office, and a web browser (well, let's be honest - it has Internet Explorer). It can do basic stuff.
But it doesn't have the software you need to do the parts of your job that make it different from every other job that involves sitting in front of a computer.
You've got to put that software on top of your fresh, pristine computer.
It's like a little kid in a room that's white from the shag carpet to the velour wallpaper.
Think of the different ways you can get your software on this computer as different toys that kid could have in their hand.
Manual install from media or download from vendor : marker
Terminal Services / Citrix XenApp (née Metaframe) : ViewMaster
App-V / Thinstall (app isolation) : Colorforms
I always did like Colorforms.