Tools have opinions, and sometimes that’s the problem

Every design tool nudges you toward certain decisions simply by making them easy. Snap-to-grid nudges you toward alignment before you’ve earned it. Component libraries nudge you toward reusing existing patterns before you’ve asked whether a new one is warranted. None of this is inherently bad, but it means the tool is making some decisions for you before you’ve consciously made them yourself.

Paper has no opinions. A pen doesn’t care if your rectangle is twelve pixels off, and that’s exactly the point — at the sketching stage, twelve pixels shouldn’t be the thing you’re thinking about. Paper keeps the resolution of your thinking matched to the resolution of the problem.

Speed is the actual benefit

We can sketch and discard fifteen layout ideas on paper in the time it takes to build two of them properly in a tool. Most of those fifteen deserve to be discarded. Finding that out cheaply, before any pixels are pushed, is the entire reason this habit has survived every attempt to replace it with something more modern.