This is an old idea which goes back to the middle of last century when horses were fed large amounts of chaff (chopped straw with molasses) (30 to 50 % of the daily ration) but were not drinking enough water (buckets instead of automatic drinking troughs).
Some horses were prone to developing kidney problems caused by sugar crystals present in the molasses (mainly beet molasses).
However, this is no longer a problem today since all horses are provided with sufficient water (mostly automatic drinking troughs), and rations consisting of chaff mixed with molasses have practically disappeared.
Moreover, sugar cane molasses are mainly used, with higher purity in microbiological terms (no soil residues) and very often mixed with soya or linseed oil which have interesting properties (omega 3).
Balanced feeds frequently contain between 1.5 and 3% molasses; as well as making the feed more appetising (horses prefer sweet to bitter tastes), the molasses content helps pellets to retain their, shape thereby avoiding disintegration when handled and, hence, limiting dust harmful to your horse’s eyes and respiratory function.