Leviticus 2:1 NKJV
‘When anyone offers a grain offering to the Lord, his offering shall be of fine flour. And he shall pour oil on it, and put frankincense on it.
The grain offering had to be anointed with oil. It had to have frankincense on it. The addition of frankincense shows the offering had to have a pleasant smell.
Spiritual parallel: When we bring offerings to God, we should see it as anointed – sanctified, special. We should have a pleasant attitude. Paul talked about this.
2 Corinthians 9:7 Amplified Version
Let each one give [thoughtfully and with purpose] just as he has decided in his heart, not grudgingly or under compulsion, for God loves a cheerful giver [and delights in the one whose heart is in his gift].