Take for example, Tamil Nadu Express. While arriving NDLS it has a stop at NZM. One of the reasons might be that while the train is reaching NZM, the busy NDLS station is alerted to arrange a platform. A small halt gets that precious time. This also turns out to be helpful for passengers.
Everything has pros and cons. In this case, it might be saving fuel and pollution that can be caused from resultant road traffic. And Delhi is already congested. Some trains donot follow such system like Rajdhani which is totally fine, by the way!