We don't know for certain what it says about St. Timothy's contribution to our New Testament -- there are too many possibilities. But E. Randolph Richards, in his Paul and First-Century Letter Writing, says,
Whatever the role of named cosender, it went beyond courtesy, presence, connection or favored status. I must conclude that a named cosender, at least after the early letters, had a different -- presumably larger -- role than the other team members. Otherwise, I see no explanation for some team members being in the letter address while others are in the concluding greetings, especially when Timothy appears first in the address, then in the greetings, then back again in the address.
[E. Randolph Richards, Paul and First-Century Letter Writing, IVP Academic (Downers Grove, IL: 2004) p. 105.]
According to tradition, St. Timothy was first bishop of Ephesus and was stoned to death when he tried to preach the gospel at a religious festival devoted to the goddess Artemis. St. Titus became the first bishop for Crete and died of old age.