The role of the pattern on the stainless steel manhole cover?

The current manhole covers are designed with various patterns on the surface, the purpose is for safety and non-slip. Walking on the street, if you observe carefully, you will find that the surface of the stainless steel round manhole cover is covered with patterns, there are crosses, herringbone, and other patterns, etc. Then there is a question, why must the surface of the manhole cover be printed? What about these patterns? Wouldn't it be better to make it smooth? The surface of the manhole cover has a unique anti-slip pattern, which is mainly to increase friction and prevent pedestrians from slipping due to rain, snow and ice. Just imagine, a smooth iron plate, when it rains or snows, it is easy for pedestrians to slip a lot when walking on it. If there is an elderly man walking on it, the consequences will be even more unimaginable.