It gets used. Because this secret chat does not save its history, it leaves no trace on the devive. So people who want to cheat on their partner, but know that their partner checks their mobile at times even installed telegram because of this feature (and they do not know what e2e means).
Surprisingly many people have this motivation. So many, that I know of persons who got problems with installing telegram, because their partner assumed - it was for this reason.
> I don’t quite understand what the point of telegram is then?
It's UX is far superior to any cross-platform competitor. On top of that, its client is open source and the servers have easy and free bot APIs. It also has channels, which broadcast your messages to followers and many secure competitors don't have such functionality.
It's not encrypted, but as it turns out most people don't really care about that in practice. If they would, texting wouldn't be popular in places like the USA. That's probably why people go to Telegram instead of Signal, ease of use is more important than privacy for most, especially in group chats that are generally about what's for dinner.
As far as I understand: many people use Telegram channels as a sort of Twitter-like social media, since you can set the channel to publicly available and anyone with app can read what you wrote, and even interact with you through, like voting through giving your post an emoji or commenting on it (if you enabled comments).
Channels are extremely public, in the sense that you can access them even without installing telegram [1] and there are websites which can generate RSS feeds for all the posts of a channel or things like that [2]. So this is not exactly a private conversation we're talking about, more like the printing of broadsheets to be nailed to every other house's wall.
At the moment, ads are only displayed in public channels with at least 1.000 users, so they are still pretty rare to come by (like, I use Telegram regularly and only see them every other day). Plus, I've heard that some clients aren't supporting them yet (even if Telegram's stance is that every client should), so maybe you'll never see them.