Okay, lets imagine u need an assembler to be placed. But your hotbar is already filled with items and you dont want to change your well organized hotbar(and it also does not contain an assembler in it). Then you have to have to open your build menu, select the assembler and then place it, which is taking a lot of time to do.
Now my theory is that, if you have a pick factory button, then you can automatically have that factory blueprint directly in your hand with the press of one button. You dont have to open you build menu to access the blueprint. All you have to do is look at the particular factory you want to build, and press the button, the factory blueprint will appear in your hands.
Hope you understand my crude explanation.