Annular isn’t really noticeable without looking directly at the sun. It’s gets much darker, but dark adaptation and no funky colors just seems relatively normal. As for every 18 months, yes somewhere on the earth is an eclipse, but they’re not always total, not always over land, not always over countries with high research budgets, and not always over English-speaking countries, which is relevant for visibility on English-speaking sites/subsites
Edit: the paper specifically talks about prior eclipses across the world. So they’re just expanding the knowledge. So if you ask why they’re doing this, why do we do anything?