Radials - and other's with inverted cylinders - are prone to hydraulic locks: When not running oil seeps past the rings into the cylinder. The fluid doesn't compress so instead of the poor piston/conrod shoving against a compressing gas during start, it slams into the fluid. End result is broken internals.
Pulling it through before start lets you feel the hydraulic lock without breaking anything. If found it can be drained by removing the plugs.