Why the most interesting problems live where a mockup stops being a picture and starts being a running thing.
There's a specific kind of problem that only shows up once an interface is real: the empty state you didn't design, the 300ms of latency that changes how a gesture feels, the copy that overflows on a Tuesday. I've spent most of my career living in that seam.
Working across both sides means fewer handoffs, fewer 'that's not what I meant' moments, and a tighter loop between intent and behaviour. The craft is in the details you only notice when they're missing.