What is the purpose of Android's tag in XML layouts?

<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 />
</FrameLayout>
which is functionally equivalent to this single layout:
<FrameLayout>
   <FrameLayout>
      <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 />
</merge>
This is functionally equivalent to this layout:
<FrameLayout>
   <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.

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