Also for keeping valve closed while installing spring and retainer. I am now a firm believer that bringing piston to tdc is for me and my cautious paranoia the safest and effective way to remove valve springs with the head on engine. Pulling a head to retrieve a lost valve gives me nightmares thinking about it.
You might back the piston up a bit then feed some 3/8" white nylon rope in the plug hole then bring it forward to keep from dropping a valve. A foot and a half is plenty/cheap & agreed you don't want to even think about what could happen!
