When you have multiple displays, you can use the Fit to buttons to fit a document window into the available area in each display. For example, you can fit a document window containing an assembly on one display and a document window containing a part from that assembly on the other.
When you have a single display, you can use the Fit to buttons to fit one document window to the left side of the display and another document window to the right side. This is particularly useful when you have more than two documents open because it lets you arrange only two of the documents. (, in contrast, would arrange all the open documents).