Holy Tradition refers to the countless writings of the ancient Christians who spoke of the beliefs and practices of their day. Many beliefs of the Catholic and Orthodox Church stem from these writings, such as the various liturgical services, hymns sung by our church, philosophical and theological defenses of the faith, and much more. When the Scriptures are unclear about a particular issue, then we turn to Holy Tradition for an answer.
Holy Tradition is necessary because it provides historical evidence to answer various questions after the time of the Apostles. In a time prior to the Holy Bible as a defined book, you can only use the commonly read Scripture books as evidence for your case. Thus, the next step would be to see what the people believed and did to support the dogma or practice. If the answer is that beliefs and practices were common among all peoples, then the probability was higher that they were Apostolic in origin.
The first major heresy was called “Arianism”, which is the belief that Christ was not divine, but rather a creature created by God. This caused great controversy in the Church, because it questioned the very foundation of Faith: “Who was Jesus?” and “Did it matter what I believed about Him as long as I followed Him?” For the ancient Bishops, they strove to defend that Christ was the Word of God Incarnate, and that the Incarnation was necessary for man’s salvation; using both Scripture and the ancient sources (1st to 3rd centuries). St. Athanasius of Alexandria wrote the definitive treatise against Arianism called “On the Incarnation of the Word”, which defended Christ’s Divinity.
Many other controversies concerning Christ and beliefs of the Church came up throughout the years, and Holy Tradition had to be used to justify the beliefs. Among them are definition of the Holy Spirit, Christ’s two natures, his two wills, the Sacraments, the role of Mary as the Theotokos, the use of icons, etc.
Sometimes, Holy Tradition is used when Scripture is silent about a particular practice. For example, some Christian denominations reject the idea of infant baptism. The Scriptures, in all technicality, are silent on it. There are references to households being baptized, which usually imply children. Likewise, there is nothing in the Scriptures that prohibit children being baptized, since no where in the Bible does it say the Apostles gave age requirements on receiving the sacrament. Holy Tradition, on the other hand, does show that infant baptism was common in the 2nd and 3rd century by the writings of the ancient Christians who talked about baptism. Thus, by this evidence, it was considered an acceptable practice, and we do it too.
To summarize, Holy Tradition is necessary since the Scriptures only give us the fundamental basis of Christian belief and a historical account up to a certain point. Everything after Apostolic times is looked at to see if the faith and practices where consistent throughout Christian history. While not all the writings are perfect, they can still be useful to understand what the people were doing and believing during that critical age of Christendom.