We all have our favorite resource packs. But some of us, just don't, they are constantly changing packs, and sometimes one pack could have something no other pack has, so they should add a feature, to select 2 resource packs (or more) and choosing which textures you want. This includes but not limited to:
-Item textures
-Block textures
-etc...
Because not all of us have the technical expertise to do it by ourselves, (or, are just lazy)
We can "extract" the specific texture in-game and put it into another pack we create. And we can rename the pack and change its icon so we can share it!
Example:
The block textures of Pack 1 is better than Pack 2. But the item textures in Pack 2 is better. So we use the Resource pack creator to create a new pack, and by "selecting" which texture/category (item, block, etc..) from both packs to put it in our new pack: Pack 3. And we can change the packs name, description, and icon and immediately upload it (or export) so we can share it.