TortoiseSVN allows you to specify your own tool for showing diffs and/or merges. For example, a lot of people prefer WinMerge over TortoiseMerge ( even though I can't see why :-) ).
But sometimes you might want to prefer TortoiseMerge over your custom diff tool. Changing the settings would be way too complicated to just use TortoiseMerge once, for this particular situation.
TortoiseSVN allows you to use TortoiseMerge easily, without changing your custom diff tool setup: just hold down the Shift key when you click on the context menu to do the diff. If the Shift key is pressed when doing a diff, TortoiseSVN always uses TortoiseMerge. If the Shift key is not pressed, the diff/merge tool specified in your settings is used.