Thoughts on a lathe gib

[Highpower]

As far as I know it is the same reason you find scraped 'ways' on a mill or on a lathe bed. Hand scraping remove high spots  to make the surface flat but also leave slight pockets to hold an oil film to keep things moving smoothly. Now think about a set of precision gauge blocks that are ground perfectly smooth and flat. When you 'wring' them together, they don't let go! There is no room for air or an oil film between them to slide against each other so they get stuck together. Not what you want to have happen on a machine.

Another procedure, 'flaking' takes it a step farther to add even more areas to hold oil.