The freshly opened transparent low-fire glaze on the left has a specific gravity of only 1.34 (thus has a high water content). Yet it is viscous and holds in place because they add a lot of gum. It needs three coats to go on thick enough and takes quite a bit of time to dry each one. Highly fritted transparent glazes need to be applied thin enough to be clear, thick enough to be glassy smooth but not too thick (to avoid clouding). Amaco likely targets the low specific gravity to enable good control of thickness. The center Potter's Choice glaze, is 1.52. And thus goes on nice and thick with each coat. That glaze likely contains lots of clay so little or no gelling agent (e.g. Veegum) is needed. The Celadon glaze on the right is in between, 1.46. Glaze manufacturers can produce at a broad range of specific gravities, they adapt the percentage of gum (e.g. Veegum and CMC gum) to impart the needed rheology and brushing characteristics.
Amaco V-303 Terra Cotta underglaze was applied in one coat, at leather hard stage (on these Plainsman Snow mugs). It appeared to be thick enough on application and after bisque (left) but the clear overglaze on the right has difused into it to melt it enough to lose the opacity. A higher specific gravity (lower water content) would make it possible to apply it thickly enough in one coat, this is especially important because each layer rewets the piece and adds a lengthy drying period. An appropriate specific gravity is thus the difference beween practical and impractical.
Commercial hobby brushing glazes
These are an incredible benefit to pottery beginners and pure hobbyists. But they can also be an obstacle to progress and affordability as your skills improve.
Hobbyists and increasing numbers of potters use commercial paint-on glazes. It's convenient, there are lots of visual effects. There are also issues compared to dipping glazes. You can also make your own.
In ceramics, the specific gravity of slurries tells us their water-to-solids ratio. That ratio is a key indicator of performance and enabler of consistency.