The recommendation for using a certified letter is that you (as customer) have an independent paper trail to make your case should it go to court. At least in NL, a certified letter should not be required by the company itself.

In Finland cancelling rental contract can be fun, if you don't manage to contact your landlord. Your regular certified letter technically isn't enough. You need even more expensive version "registered with advice of receipt". Which is probably only way to prove in court that person received it...

Though I haven't had issues in cancelling stuff. Online services work nicely for all other stuff.

... and if we want to get even pickier (again at least here in Italy) a Law firm will likely send you not (still by certified mail with receipt) a "normal" letter (i.e. one or more sheets of paper inside an envelope) but rather a "piego" (literally "fold") i.e. the sheets of papers folded in three, with the address (and the stamp) written on the back.

The rationale is that you could claim that you received the letter, but upon opening the envelope you found just some blank sheets, with the piego there is no way to deny that it has been received.

And viceversa, there have been cases of envelopes sent intentionally with blank sheets inside, only to get the receipt and then be able to claim that "document X" has been sent within a required deadline (and actually fabricating the document later).