Simple, unsophisticated, and makes it more like sketching.
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Pro Smart Snaps
Keynote has alignment grids (aka snap to grid feature) that snaps to end, midpoints, baselines etc of objects and grids. They are super useful when you want to follow a structured layout for your wireframe.
Pro Imports nearly every type of asset
Can import all kinds of assets (images, videos, PDFs, sounds) into mockup without hassle.
Pro Animations and transitions
Keynote currently is one of the very few tools that supports animation of elements and transition of views without programming.