Geometry nodes are the only nodes that does not update your material in real time when you edit/set them up. After any edits to the material you make you must "execute geometry" before you can see the effect in the Real-Time View. This can be done in a number of ways.
The Execute Geometry Node button in the properties panel will only refresh the current material.
The Geometry Nodes button in the Material Graph ribbon will refresh all geometry nodes in the scene.
The hotkey Alt+G will refresh all geometry nodes in the scene.
In the Real-time View an indicator in the top right corner will show that the scene contains un-executed geometry nodes. Click it to refresh all geometry nodes.
If you want to bake your displacement into the geometry, you can export your displaced object and re-import it into your scene.
Known Issues and Limitations
Bubbles, Displacement and Flakes are not supported in Render Nurbs Mode
When applying Displacement to geometry with hard edges, these edges become split.
When applying a single Displace node to multiple sub-materials of a Multi-Material, some sub-materials may become corrupt.