Crystals are pretty, and in general their flaws and relationships act as convenient sources of "chaos: imagery and objects arrayed in a way that does not correlate to anything in particular", which can act effectively as a seed for creative processes.
By mixing in colors or elements of the context which can cue the viewer emotionally -- such as learned, arbitrary associations between colors or materials and emotions, and in some respects evolves, visceral reactions to these things but which are no less arbitrarily evolved -- the viewer of the crystal can select a genre of thoughts to apply this source of chaos to.
This chaos is then used as a basis to form new math and understandings in the mind: chaos is useful for pushing the mind into new configurations.
Sometimes, crystals can be useful in other ways, to present various forms of stimulus to the mind, as a distraction while again being useful in their color and material as a cue, a piece of vocabulary that for me with a particular piece of amethyst with some cracks in it makes a good "distraction object".
And indeed focusing on these things can be a good context reminder of letting yourself meditate on specific problems, while also being able to leave that problem behind with the crystal while you are not free to meditate on it.
Notice that none of these very real uses for crystals require one single iota of spiritual connection anywhere but one's own imagination, though clearly one can accept the popular syntax around them and apply them as suggested to have a leg-up on developing the "vocabulary of use" of those crystals.
I would suggest that this is the most useful and sane way to apply crystals in one's practice.
As always, if someone is experiencing a material illness caused by some material configuration within their body (and I see all illness thus), the correct course is to take the material remedy for what aisle: chemo for cancer, not crystals.