When you inspect the handle of a classic safety razor, you may take note of aspects like the heft that comes from solid metal parts, or the length of the tool and its fit in your hand. For many safety razors, though, you'll feel a very rough pattern cast or engraved into the main section of the handle. This tactile pattern is called knurling, and has been a traditional element of classic shaving tools since their inception.
When going through all the proper steps of classic shaving, not only your face but your hands and safety razor may be covered in slippery substances like shaving oil or shaving soap. The knurling provides additional grip when you need it most, helping avoid slips that could cause cuts. Knurling is helpful, but with proper rinsing between each pass of the blade, you should be able to keep slipping to a minimum even with a smooth-handled safety razor.
If slipping is a problem for you when wet- shaving, you should consider switching to a traditional Merkur safety razor with knurling on its handle, increasing rinsing between passes, or both. Additionally you may want to have an alum block on hand just in case you need it. Now go forth and enjoy a better, safer shave thanks to your safety razor's knurling, or at least a little bit more knowledge about your shaving tool!