If you should err and not fulfill all the commandments that G-d spoke to Moses.
Num. 15:22
The commandment to set aside part of our bread for the priest is followed by a discussion of idolatry. The reason for this is as follows:
The reality of our physical world is that we have to work hard to satisfy our needs. Because of this, it is easy to fall into the trap of feeling that our material success is dependent on the brutal laws of nature: the more and better we work, the more we earn. It is easy to feel that G-d is not involved – and this is a subtle form of idol worship. In truth, the forces of nature are only tools in the hands of the master Craftsman; they have no more influence on the world than a craftsman’s tools have on his work.
In order to avoid this misconception, we are told to set aside some of the very first of our dough as a portion for G-d. This reaffirms our faith that it is indeed G-d who has granted us all that we have and that He is truly the source of our sustenance.
— Daily Wisdom 3