Friction is indeed a force that resists motion between two surfaces in contact. However, friction also plays a crucial role in facilitating motion in many situations. Let's explore this further:
Starting and stopping motion: Friction helps in initiating and stopping the motion of objects. When you push a stationary object, the force of friction between the object and the ground opposes the initial motion. However, as you continue to apply force, the friction gradually decreases, allowing the object to start moving. Similarly, when you want to stop a moving object, the force of friction helps slow it down and eventually bring it to a stop.
Traction: Friction provides the necessary traction between surfaces to prevent slipping. For example, the friction between the tires of a car and the road allows the car to accelerate, decelerate, and make turns safely. Without friction, the tires would simply slide on the road surface, making it extremely difficult to control the vehicle.
Walking and running: Friction between our feet and the ground allows us to walk or run. It provides the necessary grip and stability, preventing us from slipping and helping us propel ourselves forward.
Transfer of energy: Friction converts kinetic energy into other forms of energy, such as heat and sound. While this conversion may seem like a loss, it can be advantageous in various situations. For example, friction between our hands and a rough surface generates heat, which can be useful when starting a fire by rubbing two sticks together.
Regarding your second question, in a vacuum where there is no air or any other medium, there is no air resistance or fluid friction. However, other forms of friction may still exist depending on the context. For instance, if you have two objects in contact, there might be friction between them due to intermolecular forces or microscopic surface irregularities. It's worth noting that in a vacuum, without any external forces, an object will continue to move at a constant velocity once set in motion (according to Newton's first law of motion) since there is no friction to oppose its motion.