Generally there is a widespread population deficiency of Mg and K in modern diets. K can be discussed separately.
Mg occurs at about 3% in sea salt and is what tends to be leftover after the Na precipitates out : traditionally used to make tofu.
Resultingly MgCl2 is relatively cheap, compared to modern packaged suppements. It is stored as a hygroscopic salt, but the water doesn't hurt the function.
Supplementary administration is generally limited by what the gut can absorb without laxative effects, typically 0.35g Mg in a single dose. A tiny 0.1g dose of Mg2+ will be only 0.4g of the salt, about the size of a sweetener tablet. This is also sufficient to alter the flavour profile of a cup of coffee, towards reduced bitterness, and may be the simplest way to incorporate it daily.
The central and motor nervous system effects, we can leave to your discovery. Typically people find it relaxing and helpful for sleep, including promotion of dreaming