Why be biblical? Because the Bible communicates the gospel of Jesus Christ--the account of what God has said and done as sovereign Lord and as suffering servant for our salvation.
"Evangelical" designates theology that seeks to know the "God of the gospel," the God who revealed himself in the prehistory and history of Jesus Christ as well as through the apocalyptic vision of his future coming. I am an evangelical theologican because I believe the "good news" of what God has done in Jesus Christ and because I believe that theology--indeed, all of life--ought to be conformed to this reality, for ultimately there is no other reality, just as there is no other gospel (Gal 1:7).
Kevin J. Vanhoozer, "The Voice and the Actor," in Evangelical Futures, p. 63.