Here on earth, things are continuously affected by the force of gravity and friction (air resistance, water resistance, rubbing against a solid surface, etc.) To keep something moving here in earth, therefore, you have to continuously apply force to counteract those ever-present forces. That’s why it seems so natural for things to be that way.

In the absence of gravity, friction, and any other forces, however, an object will continue to move in the same direction and at the same speed until a force is applied. In fact, it would actually require some sort of force to make it slow down to a stop.

I don’t know how much science fiction you may have read, but there’s a fairly standard theme with sub-lightspeed spacecraft voyaging from earth to nearby stars. They constantly accelerate and build up speed until they’re at the halfway point, then they flip around and accelerate in the opposite direction in order to slow down enough to stop when they arrive. If they simply accelerated in the same direction the entire time, they would overshoot their destination with no way to stop.

Hope this helps!