What is the importance of foot dust?

The scriptures have emphasized special significance of foot dust for devotees on the path of devotion.

It is stated in Chaitanya Charitamrita:

भक्त पदधूलि आर भक्त पद जल भक्त भुक्त अवशेष तिन महाबल,
एई तिन सेवा हैते कृष्ण प्रेम हय |

"The food dust, foot wash and food remnants of a devotee of Shree Krishna has such agreat spiritual power, that an aspiring devotee can easily obtain the divine love of Shree Krishna, through the grace of the Guru, by regularly using these as prasad."

 
Thus, the foot dust of the lotus feet of a Guru and Shree Krishna are extremely important in the process of devotional practice and on the path of devotion.  By using and consuming this as prasad, one can cleanse his heart and mind quicker, thereby become deserving soul of the divine love of Shree Radha Krishna sooner.