A lot of information is available on the internet about Sankranti, its relevance and religious signficance. The above podcast talks about why we eat jaggery, til, sugarcane and groundnut based food items on this occasion.
As the postcast is in hindi, I have translated the gist of it in English here.
Why eat jaggery on Sankranti
Jaggery, sesame seed, groundnut, sugarcane, coconut and various similar types of food items are eaten on the occasion of Makar Sankranti to detoxify and cleanse your liver and provide overall warmth to your body. During this coldest period of the year, eating these food items in the name of festival is essentially a traditional way to enjoy and keep your health in good shape.
Sankranti