Why is fuel needed to change the speed or direction of a probe in space?

Fuel is use to produce a force to accelerate in a new direction. In the void of space the object will continue its velocity until somthing acts upon it. (newtons first law of motion)

Newton's first law of motion clearly states that once an object is traveling in a direction, that object will not change its direction or speed until something slows it down, stops it, or makes it change direction. Outer space creates a void where using a fuel propellent is the only way to move a probe.