The draw of an idle game really changes from game to game. Most of them, like AdCap, pretty much lack any depth in their gameplay and mainly work because of their addictive nature - which, along with their relative ease of making and their length which is a consequence of the genre, is the reason they're all over the place on most gaming sites' frontpages.
99% of an incremental game's gameplay is an optimization puzzle; it can range between a run-of-the-mill idler's "There's this upgrade that costs $5e43, which should take about half an hour to reach at this point in the game, giving a +150% boost to production. I'll skip the other upgrades, as they're not worth the cost", to something much more complex that can come from entirely new features being introduced, like "the Lineage feature turned the Undead Bloodline into a ridiculously strong upgrade that's the key to progress at this stage, but it comes at the cost of not having the great mana boost from the Druid Bloodline, and it requires me to build up about a quintillion faction coins each run, but they aren't a resource I've made a focus of my previous builds - lemme go back to the drawing board for a sec..."
Of course, games which feature the second kind of gameplay features are much less common, as most devs settle with the "quick and easy source of ad revenue" option - if you want good examples of quality incremental, I'd recommend something like Trimps, Kittens Game or Realm Grinder (which is incidentally the one I took my example out of)