A recurring user request is to be able to have a widget in the plasma panel which is always exactly centered (as a reference, the Clock of Gnome3, recurring request for the taskbar as well)
Right now is possible to approximate this with spacers in the plasma panels, but is not possible to actually ensure that the applet wil lalways be exactly centered, as this depends from the space that widgets on the left or on the right will take.
Latte dock has an approach of having 2 spacers always present in the panel (they can't be neither added nor removed) tough this causes some ui problems: it makes impossible for applets to actually take all the available space anymore, and is a quite weird UI.