Keep specific projects tied to exact Editor versions on a single drive for consistency.
Run Unity directly from your drive without leaving registry clutter on the host machine.
Carry your entire development environment, including assets and scripts, in your pocket. How to Create a Unity Portable Install
While Unity doesn't offer an official "portable.exe," you can create one by following these steps to consolidate the Editor files: Unity3d portable installation - Unity Discussions
Perfect for school or office computers where you cannot install software to C:\Program Files .