What would be the reason for selecting the portable over the regular version of LibreOffice for use on my laptop and desktop W7 computers?

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Let me add one more comment to the excellent explanation by @Lupp

In the case of presentations I am also often asked to provide a ppt or pptx copy as a fall back solution is something happens with my PC or connection of my own PC is not possible, etc. For this I always carry a USB stick with a portable LibO version on it and my presentation. All I need is Windows PC to plug the USB stick and can run on LIbO my own presentation.

Portable apps are a kind of packaging and making available applications on Windows systems without an installation. The windows registry will remain (nearly) untouched. Portable apps are often used together with the PortablApps platform. Also they are often placed on a removable/portable storage device and can then be used, via USB mostly, on any windows host (nearly) without leaving any residues. Many versions of LibreOffice are available as portable versions packaged by the PortableApps people or by the document foundation itself as well. Read more here.

All this is completely independent of whether we talk of a "desktop" or of a "laptop / notebook" computer.

The one fact many users see as a disadvantage of the portable versions is that they are not by installation linked to some file extensions as the standard application to open the files with. You have to first start the application and then open the workfiles. If a portable app ist unpacked on a system at a fix point of the file system (unchanging "driveletter" e.g.) you may, however, associate it with extensions subsequently.

So do I. I installed LibreOffice on all my PCs and use a USB pendrive when I need to use someone else computer.