Stepping on the soil, even in dry weather, tends to compact the soil, and force air out of the gaps between the soil particles.
This is one of the main benefits of raised beds - they define a boundary to the growing area, and deter people from walking on the soil.
If you step on the soil when the soil is wet, this compacts the soil even more than it does in dry weather.
Some people use to plank to stand on when doing any cultivation, as this spreads the weight of the gardener over the entire area of the plank, and so reduces the pressure, and the compaction.