The nets are much more successful in capturing all sorts of birds at once, with little effort, even in environments where you wouldn't get clear line of sight for photos and/or for species that don't sit still for long enough.

Handling the birds makes for far better identification (and more detailed pictures too), which is important for places where unknown, hard to identify or hard to detect species are likely to exist. It also allows taking measurements, banding the birds and even collecting tissue samples. So nets are a far better ROI for some scientific projects.