Racehorses carry extra weight so that ‒ in theory ‒ they all arrive at the finish line at the same time.
Of course, while a noble and ambitious goal, this almost never happens because this is racing and a lot of influence is inflicted by the track conditions, the weather, the distance, the jockey, the company, the horse’s fitness and even how it is feeling on that particular day. But the idea of weight assignments is to give the horse, its connections and the betting public the fairest chance of finishing in the money.