Most often, it is necessary when a file is modified by two people on two different computers at the same time. When two branches are merged, the result is a single collection of files that contains both sets of changes.
In some cases, the merge can be performed automatically, because the changes do not conflict. In other cases, a person must decide exactly what the resulting files should contain. Many revision control software tools include merge capabilities.
Merge can be used as a verb ("to merge branches"), but can also be a noun ("this merge will be difficult").
Merging multiple data sets is another use of this tag.
Merge is different from join in datasets which is primarily SQL based. Difference between merge and join for SAS is here http://www2.sas.com/proceedings/sugi30/249-30.pdf and for R is here How to join (merge) data frames (inner, outer, left, right)?