if I may add some complement to the previous answer, delta represent the time difference (time delta) between this frame and the precedent one.
As process execute as fast as possible for the PC (depending on its CPU processing power) the rate of execution between a PC and another (or even on the same computer but in different situations) may vary.
So when you multiply a
speed_factor by delta in a
_physic_process callback, the
speed_factor is now a unit / second
position = transform.x * 200 * delta in a 2D game will move the transform forward by 200 pixels per second
_physic_process has a stable
delta which initialy is 1 / 60 (to match 60FPS) however, this can be configured in the "Project" menu
Hope you better understand why your code runs differently using delta or not ;)