The auto-population is both a blessing and a curse. When it works, it works well. For example, if you search for a location, that location will be saved as a card that you can go back to later — including the full details on the transit time. The same applies to weather, sports scores, flight times, locations, and a few more bits. Cards will also appear both inside Google Now and in your notification area when called for — like when you're standing at the bus stop and your bus is late.
The downside to this auto-populate method is that you sometimes feel as though you don't have direct control over your cards, why they appear, and how to get them back after you swipe them away. Each card had its own settings where you can choose a "priority," which in theory should help determine where the card appears in your stack. In practice, however, we'd have much preferred more direct interaction and control.