No, Jesus wasn't just trying to cheer up His followers by promising them that He would come back to earth some day (although I'm sure His words did encourage them, in light of Jesus' impending death).
Instead, Jesus was teaching them a very important truth -- and one we need to take very seriously. The truth is this: Some day, all evil will be destroyed, and God, not Satan, will be victorious. God alone is sovereign, and even when it looks like Satan is winning (as it did when Jesus was put to death), his "victory" is only temporary.
Yes, some day the final battle will be fought between good and evil... between God and Satan... between the powers of light and the powers of darkness. And God will be the victor in that battle, and Christ will come again to establish His Kingdom of perfect justice and love. Some day, the Bible says, the angelic voices will proclaim that "The kingdom of the world has become the kingdom of our Lord and of his Christ, and he will reign for ever and ever" (Revelation 11:15).
In a world of turmoil and uncertainty we can have hope -- hope for this world, and hope for eternity -- because of Jesus Christ and what He has done for us through His death and resurrection. Is your hope in Him? If not, turn to Him in repentance and faith today.