It's to do with the way the compressors in fridges and freezers are made these days. If they've been tilted too far from the vertical, the lubricating oil inside the compressor can get into the coolant gas pipe, and block it. Letting it stand for a while allows any such oil to drain back into the compressor before you switch on. If the manufacturer says this will only take half an hour, then that should be OK.
Old fridge freezers didn't have this problem, but that's progress for you!