How to use visual merging tools?

I can't get my head around KDiff3 or Beyond Compare in the merge mode. Is there any tutorial to this feature?

What is the use of merge tool?

What Is the Merge Tool? The Merge Tool combines data from multiple sources, then adds them into a new data set. It's not only geometry, but it also merges attributes with the option to match fields from input datasets. When you use the Merge Tool, features have to be the same geometry type (points, lines, or polygons).

How do I start a git merge tool?

Use git mergetool to run one of several merge utilities to resolve merge conflicts. It is typically run after git merge. If one or more <file> parameters are given, the merge tool program will be run to resolve differences on each file (skipping those without conflicts).

Here's a video showing how to use Beyond Compare to merge files. It was given as part of Embarcadero's CodeRage 5 conference, so it covers a lot of general usage as well. The merge part begins at about 13:00.