The link is german, so here’s the important bit translated:

It will soon be possible to mark the device you use to log in to WEB.DE as a “known device”:
Your device will be registered by a confirmation via the WEB.DE Mail App or with a code that you can request via SMS to your cell phone.
Once saved, the system recognizes your device the next time you log in and offers you an additional security factor when logging in.

This just screams Let’s get people who don’t know any better to give us even more data about them.
(Their site already shares data with over 280 pArtnERs, btw)

I they really cared about login security, they’d offer bog-standard TOTP for this purpose.