For me it's the combination of:

1. Multimedia. I have a cookbook with photos and videos in my Obsidian, as well as songbooks with musical notation that renders properly.

2. Databases. e.g. for an international vacation I created the itinerary as a "Base" (structured database) that rendered out to a calendar and day-by-day pages. This even included floating itinerary items like "eat X kind of cuisine" to squeeze in on any day, shopping lists for gifts to bring back, as well as all train routes, flight numbers, hotel/ticket confirmation numbers and the like. Then on each day I could journal what we did and see what we still had to do. I was able to share these with everyone on the trip and make sure everyone got to experience their must-see items.

3. You can argue all of the above would also work as a website. But Obsidian makes this work across my phones and tablets too, in a way that I can edit from mobile whenever I need to jot down a quick edit. That's the killer feature.