Because every gcode file that I've ever seen specifies the linear feed rate in mm/min:

G1 X134.898 Y125.559 F1500.000 ; move to first perimeter point
G1 X134.678 Y125.367 E134.06120 F1500.000 ; perimeter
G1 X134.434 Y125.228 E134.07645 ; perimeter
G1 X133.641 Y124.383 E134.13941 ; perimeter
G1 X133.437 Y124.052 E134.16051 ; perimeter
Does your experience differ? Please share. Maybe it's just our firmware flavo(u)r that uses mm/min.

That's a positioning light, so you can see what area your work will occupy.