An icon is kind of “visual shorthand”: a way to leverage peripheral vision and pre-attentive processing so a user can understand a control's purpose in milliseconds.

The user won't learn a UI from icons alone, but once learned, distinct icons speed up recall massively. The problem with the modern trend of hyper-homogenised, uniform icons is that they destroy this advantage. When every icon has the exact same stroke weight, color, and geometric bounding box, they blur into a useless mush.