The modak is the favourite sweet of the Hindu deity, Ganesha. It begets him the moniker modakapriya (one who likes modak) in Sanskrit.
During Ganesh Chaturthi, The puja always concludes with an offering of 21 modaks to Ganesha as prasadam. Modak's made with rice flour shell are often preferred for this purpose, although, wheat shell version are also used. Local businesses outside Ganesh Temple's across India usually sell pre-packed/readymade version of Modak's.
complete ganesh chaturthi recipes here