This is a defensive movement used to avoid a jab or cross coming from your opponent
Combination using the slip
A good habit to develop is using a technical drill for sparring. In this case, to practice the slip, try a few rounds where you and your opponent can only throw jabs and after every jab you must slip or duck, this is a great way to ingrain this into your movement to make it automatic. As the most oft-used punch, it stands to reason the jab will also be the most oft-countered punch so learning to move your head after throwing the jab is essential to your success.