I think it's meant to be a subtle addition to overall feel when user lands on the page. I've not seen this happen in SaaS products, however, most product marketing/landing pages have a certain "uniqueness" to create a brand/product perception by employing such effects. One of the examples I can think of is Stripe, all the mix of graphics and animations on their product landing pages adds a bit to the overall feel. One other community platform that stands out from design point of view is IndieHackers. Ofcourse, HN has its own charm.