Charles H. Spurgeon on Psalms 119:108

Freewill offerings. This expression is often used in the law (Leviticus 22:18 Numbers 29:39 1Ch 31:14 Amos 1:4-5). What are these freewill offerings? They are distinguished from God's stated worship, and distinguished from that service which fell under a vow. Besides the stated peace offerings, there were certain sacrifices performed upon certain occasions, to testify God's general goodness, and upon receipt of some special mercy; and you will find these sacrifices to be expressly distinguished from such services as men bound themselves to by vow (Leviticus 7:16)... These serve to teach us two things.

First. They are to teach us how ready we should be to take all occasions of thankfulness and spiritual worship; for, besides their vowed services and instituted sacrifices, they had their freewill offerings, offered to God in thankfulness for some special blessing received, or for deliverance from danger.

Secondly. It shows with what voluntariness and cheerfulness we should go about God's worship in the Gospel, and what a free disposition of heart there should be, and edge upon our affections, in all things that we offer to God; in this latter sense our offerings to God prayer and praise should be freewill offerings, come from us not like water out of a still forced by the fire, but like water out of a fountain with native freeness, readily and freely. Thomas Manton.