The forward slip can be a useful skill to master since it allows for fine-adjustment of the airplane’s height above the runway on final approach, allowing you to stay on glide path while maintaining your target approach airspeed.
Slips are useful when you’re high and need to lose extra altitude in a hurry to make your touchdown spot. In these cases, it makes little sense to just lower the nose to get down to the runway since that will cause a corresponding increase in airspeed. The skill you need to hone is the ability to dissipate the extra altitude without increasing airspeed above that required to land.
