Title says it all. Under what circumstances would 8.1 Pro be better than standard 8.1, and for what reasons?
Here you go http://en.wikipedia.org/wiki/Windows_8_editions
Windows 8.1 comes in three different versions: Basic Edition, Pro, and Enterprise.
Basic Edition – Windows 8.1 Basic Edition (or just Windows 8.1) is intended for home users. This version includes the core features, but none of the business features. Basic Edition is great for those general consumers (mother, grandmother, father, step-uncle, far removed cousin).
Pro – Windows 8.1 Pro is the operating system intended for small and medium-sized businesses. It provides abilities such as joining corporate domains, participating in Group Policy, allowing involvement in Work Place Join and Work Folders, encryption and more. You can find a good list of business features in the The IT Guide to Windows 8.1. Note that some of the new business features in Windows 8.1 Pro require a server running Windows Server 2012 R2 somewhere in the organization.
Enterprise – Windows 8.1 Enterprise is the version that brings business premium features to Windows. Available through Windows Software Assurance, Windows 8.1 Enterprise includes all of the same features of Windows 8.1 Pro and then adds things like Windows To Go, DirectAccess, BranchCache, AppLocker, Virtual Desktop Infrastructure (VDI), and Windows 8 app deployment.