Whether one believes in Sola Scriptura or not, the Bible remains a reasonable source of Christian teachings.
Also, material not directly found in the Bible cannot be avoided by any church, especially in the long term, since religious leaders have to say something. No one wants to simply repeat the Bible. Everyone wants to make comment about it. These comments eventually become part of that church's heritage.
When these commentaries are documented and organized, they can (and some do) become the official teachings of that church.
Tradition is another matter that cannot be avoided by any church in the long term. Any group eventually develops its own culture (or subculture). They will eventually form their preferred or familiar way of understanding and doing things. Culture held long enough can be considered tradition by the succeeding generations.