Nowadays in America we call them pharmacists. Back when these neat tokens were in widespread use, they were called apothecaries -- people who formulated drugs. Apothecaries would use them in various sizes:
20 grains = 1 scruple = 1.296 grams
3 scruples = 1 drachm (dram) = 3.888 grams
8 drachms = 1 troy ounce = 31.1 grams
and the weights would have neat symbols on them known easily by apothecaries, but mysteries to the rest of us (they come from ancient Rome!).
There is a neat article on these collectibles over at Forrest's Token Page.
Value wise, weights in good condition pick up some nice value. But most apothecary weights are worn, stained, and generally ugly.
worn: $2 US dollars approximate catalog value
average circulated: $3
well preserved: $5
fully uncirculated: $15
The values above are for a single weight. Often they appear in sets. Condition is everything!