Is it possible that the theory wouldn't have any practical purpose?
This is not only possible, but extremely likely. And, indeed, discovery of a perfectly correct theory of quantum gravity is an area of active research that could easily happen in our lifetimes.
Einstein's theory of general relativity is extremely accurate over a wide domain of applicability. A theory of quantum gravity would reproduce and explain the successes of general relativity in this domain of applicability, but would not materially change it in circumstances where it works and certainly in all circumstances where it has engineering relevance (e.g. GPS satellites).
There are basically only two areas where a quantum gravity theory would differ in its phenomenology (i.e. predictions for the real world) from general relativity:
The details of what is going on in extremely strong gravitational fields near black holes and the Big Bang.
An explanation for phenomena describes as "dark matter" and "dark energy" in extremely weak gravitational fields at distances on the order of the size of galaxies or larger.
A correct theory of quantum gravity might allow for the engineering of better gravitational telescopes (like LIGO today), and might allow us to better understanding of what our telescopes see in the sky, but that is about it.
A quantum gravity theory might also help guide us towards a "theory of everything" revealing the deeper structure of the laws of the universe and perhaps calculating some physical constants exactly from first principles rather than simply measuring them, but this would be an additional step beyond quantum gravity and the engineering benefit there would be mostly in the high energy physics involved in short lived particles made up of quarks (called QCD) where physical constants are currently known least accurately.
Pretty much all of the fundamental laws of physics with engineering relevance have already been discovered to the full precision at which they have significant engineering implications, outside some very narrow areas like practical nuclear fusion reactors where more precision in existing physical laws would have engineering benefits.