Warm-up is an essential part of training. You cannot be effective enough when skipping warm-up.
Warm-up gives you the mental motivation and gets your joints, ligaments and muscles ready for the hard work. If you want to reach your fitness goals, you have to avoid injuries and do your best while working out. Warm-up gives you extra energy and helps you train more effective.
It is a quick, 5 minute warm-up, so please, never skip it!