Commentaries:
The Israelite set aside the second tithe throughout the year and consumed it at the annual holy festivals of God for whatever his heart desired. This means that he spent the tithe on things that enhanced his glorifying of God or added joy to the feast.
The same applies to our use of second tithe today. Unlike the use of first tithe, the individual who saves second tithe should use it. God wants His people to enjoy the physical abundance He provides at the feasts as they worship Him and learn to reverence and fear Him in ways that please Him. Because of His blessing, many can also help others observe the feasts.
Martin G. Collins
Tithing: Second Tithe
When we give God back the tithe that is His to begin with, He blesses us abundantly spiritually and physically. Notice that God says He commands tithes so "that there may be food in My house" (Malachi 3:10). Tithes are primarily used to feed His people spiritually! If we support the work God is doing through His faithful ministers, He promises to bless us far above anything we could imagine (Ephesians 3:20-21)!
Richard T. Ritenbaugh
Tithing
By paying to God what we owe Him (that is, His tithes), He rewards us with blessings. Christians often find their third tithe years to be abundant with all types of blessings and invaluable lessons learned. These are not always material blessings, however. Storing up spiritual treasures in heaven is far more important than physical prosperity. God does not promise to make us wealthy but that our relationship with Him will prosper. Such eternal blessings are far greater than any temporary physical blessings we could receive.
Martin G. Collins
Tithing: Third Tithe