Once upon a time, with the earliest releases of Revit MEP, a deleted host meant not only some rework but lost work as well because those elements were deleted too. So imagine putting in a lot of receptacles or light fixtures and then the architect deleted a ceiling to redesign it. The next time you load their project file you'd find a bunch of stuff missing. Not good...they fixed that! Good thing!
Now when you place equipment, fixtures and devices on a host they just become disassociated with the work plane of their previous host. The elements stay connected to their circuits and no lost work. I'm not saying there isn't rework involved but the stuff you took the time to do didn't just vanish into thin air.
When these elements get disassociated you can select them and choose the REHOST button on the Options Bar.
What some miss is that you can use this tool for multiple elements, say several light fixtures at once. As long as there is a valid host "under" them you should be good to go!
A related issue is, "Gosh, I wonder what the elevation of that ceiling is...before I go nuts sketching duct work and piping?" We'll some will quickly grab their Section tool and sketch a section view to find out. That's good, they've finally gotten comfortable with creating views to explore the model as much as to put on paper. However the Spot Dimension > Spot Elevation tool is a bit handier perhaps?
As long as the view is using something other than wire frame you can place a Spot Elevation to display the elevations of the ceilings. There is an example in the first image above. We can't just tag the ceilings in a linked model so this approach is next best.