TLDR:
1. Americans have a 10% lifetime risk of an Ivermectin responsive parasitic infection, rising to ~50% if you travel to Asia/Africa/South America. 75% of those are asymptomatic so go undetected, and it's not uncommon for them to persist for 30+ years. This places a load on the immune system, raises inflammation, etc. A single dose clears most of these.
2. It can be used to improve skin condition by eliminating demodex mites, which nearly every adult is colonized with.
3. Ivermectin is, for almost everyone, harmless. In hot climates where parasites are a major problem, infants through the elderly are given annual preventative doses. Until a few years ago, it was given to every school aged child in Japan annually.