As for why important: a melange of reasons. I had heard a _lot_ from customers and prospects that they couldn't find the site because they searched for/navigated to buttondown.com; I was faintly worried about SEO/brand impact from using a vanity TLD like `.email`; I was faintly worried about a more well-financed competitor buying it and redirecting to their own property; there are a very real number of legacy pieces of software that do not _accept_ `buttondown.email` as a valid domain, which is important when you're dealing with UGC.
(When I was debating what a reasonable purchase price might be, the framing that helped me make a decision was: "if this helps my overall conversion funnel grow by 2% in perpetuity, is it worth it?", and that answer was yes. Time will tell, of course, to see where that number actually lands!)
My margins are solid and there aren't a lot of competing opportunities to deploy capital besides pulling forward cloud costs or hiring folks (which is a very different value prop altogether), so reasoning about the lump sum was pretty easy, but you're definitely right in that objectively speaking I should have just gotten a line of credit and amortized it that way.
(T/Y — fixed.)
(I would also be remiss if I didn't say that I am grateful to HN for introducing me to what was called microISVs a decade ago, "indie hacking" five years ago, and now I suppose is mostly called "building in public" / "lifestyle businesses". I was inspired to start Buttondown in no small part due to reading about Candy Japan, Appointment Reminder, et al, and learning that there was a different yet equally valid path for growing a SaaS)