Not sure myself, but I think the made up mixture starts to separate when sitting around. I like to mix it and drink immediately.
Plus I usually add cream and coconut oil to my shakes and blend it in a blender with ice cubes, so it has a nice smoothy type texture.
When using shakers at work I use the one above and have a mix of water and cream or half water and half milk, and the protein powder separately in the top compartment. But really there is nothing stopping anyone from taking a separate small container with protein powder along and just use a normal shaker.