Do many restaurants use the Köln Card?

Me and a couple of friends were planning on going to Cologne in july for a gaming event. The event got cancelled but we're still planning on going.

While researching we found the Köln Card. It looks pretty useful for the public transport alone, but it says it offers up to 50% discount in restaurants. Is this the case in many restaurants or only a few?

We're still looking for fun activities, so feel free to suggest any (my friends aren't that into history/culture though)