At one point it was gamification to tie the reward center of your brain into continuing to train using the app. Now it seems that it's half that or half https://en.wikipedia.org/wiki/Dark_pattern to get you to pay to keep that reward system going.

To make you addicted so you keep coming back

Personally, my interest in language learning comes and goes. Doing the minimum to maintain a streak prevents me from forgetting too much before my interest returns.

Gamification including social aspects if you're a regular app user.

Abusive ex if you aren't a regular user.

The only way it could get worse is if they put a slot machine or roulette wheel in to give you prizes.

Sense of achievement mainly