Throughout the VRED UI, the mouse button used changes the behavior of the action.
Numeric value fields in most modules have a complimentary interactive UI element for changing values, such as a slider or scroll wheel. Your mouse buttons are also assigned interactions:
LMB-click and enter a new value.
MMB-click and drag to scroll through the values. This is the same behavior as using the scroll wheel.
RMB-click and release to step through values.
In the Material Editor, the mouse button used determines the behavior of the dragged material.
LMB-click-drag and drop a material onto geometry in the viewport to replace the material with this new one.
MMB-click-drag and drop a material onto geometry in the viewport to replace all instances of that material with this new one. Use this to change all instances of one material to another with one action.
RMB-click-drag and drop a material onto geometry in the viewport to add a switch or Multi Pass material.
In the viewport, use the M key and different mouse buttons to select a material and assign it to different geometry.