Missions is at the core of the gospel. God's desire is that His fame and glory are seen throughout the earth (Ps 19, Rom 1). As His Church, we get the opportunity to be the means through which God proclaims His manifold wisdom to the world (Eph 3:10).
Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all that I have commanded you. And behold, I am with you always, to the end of the age. (Matthew 28:19-20 ESV)