Like everybody says, it was traditionally used in fishkeeping to reduce stress, and ward off stress related disease. A bit like feeding antiobiotics to chicken in intensive farming. Much better obviously if one can produce a stress-free environment without tonics- by frequent water changes, a varied diet and good stocking plan.
Plus now it is understood that many fish will suffer longterm health damage if living in salt. (Like it is now understood that a high salt diet is not good for humans in the longterm: nobody mentioned that when I was a child.) Particularly vulnerable are scaleless fish and fish like tetras that come from acid softwater areas. Livebearers can often cope well with semi-brackish conditions, but most do not require them.