<merge/>
is useful because it can get rid of unneeded ViewGroups, i.e. layouts that are simply used to wrap other views and serve no purpose themselves.
For example, if you were to <include/>
a layout from another file without using merge, the two files might look something like this:
layout1.xml:
<FrameLayout>
<include layout="@layout/layout2"/>
</FrameLayout>
layout2.xml:
<FrameLayout>
<TextView />
<TextView />
</FrameLayout>
which is functionally equivalent to this single layout:
<FrameLayout>
<FrameLayout>
<TextView />
<TextView />
</FrameLayout>
</FrameLayout>
That FrameLayout in layout2.xml may not be useful. <merge/>
helps get rid of it. Here’s what it looks like using merge (layout1.xml doesn’t change):
layout2.xml:
<merge>
<TextView />
<TextView />
</merge>
This is functionally equivalent to this layout:
<FrameLayout>
<TextView />
<TextView />
</FrameLayout>
but since you are using <include/>
you can reuse the layout elsewhere. It doesn’t have to be used to replace only FrameLayouts – you can use it to replace any layout that isn’t adding something useful to the way your view looks/behaves.