Can 12"x12" ceilings on ground floors typically be made solid/permanent?
That's called a drop-ceiling, or a suspended ceiling.  It doesn't have to be "justified".  It's one of several choices with many advantages and disadvantages.  If you damage other types of ceilings, you'll spend hours speckling and sanding and painting them to repair, whereas with a drop ceiling you throw the tile away and put a new one in.  They can leak air/breathe a bit, so you might not want them between a living space and attic.

A solid ceiling will usually be 'higher' than a drop ceiling because you don't need the clearance to install tiles and suspend the frame.

If you haven't looked at everything above it you should before considering replacing it with another type of ceiling.  There's probably a good reason that it's there.  Perhaps it conceals beams or ductwork.