There are of course many valuable truths and insights that are not found in the Bible. There’s not a word in it, for example, about the Pythagorean Theorem. And the fact that the Bible contains many valuable truths and insights is not proof that it is Scripture. Poor Richard’s Almanac is not Scripture, for example.
Still, the wealth and depth of truths and insights in the Bible ought at some point to make one stop and wonder whether perhaps it does contain that special sort of truth and insight that is divinely inspired and that we call Scripture. (There is of course much other evidence that it is Scripture, like the fact that Jesus cited it as such; see here.)
And that ought to encourage those searching for, thinking about, and then writing about those truths and insights. It means that their work is, or can be, evangelistic: that it can bring people to God. A reader might read an interpretation of this or that passage, or a connection between one passage and another, and think, “Wow, I can see God’s hand here.” Needless to say, I hope that at least once in a while I can help with that.