As the other answer mentions, the colt is seen as part of prophetic fulfillment, going back to Old Testament prophecies about the Messiah. It's not exactly practical, since Jesus could have walked into Jerusalem just as well and done what he needed to do.

On another level, the colt represents humility, setting Jesus apart from a typical worldly king. One would normally expect a king to come in on a stallion or something impressive. In contrast, a colt is considered a beast of burden, suited for manual labor and not considered the most attractive animal to look at—hardly the ideal vessel to bring a king into a great city. But Jesus chooses a colt, emphasizing his separation from the world's definition of kingship and foreshadowing his own humility during his death.

The idea that the colt was to be used to help Jesus get the word of God to more people is not the most common interpretation of this passage in Luke.  Instead, Biblical scholars generally believe that this is another way to emphasize that Jesus is the Messiah.  When Jesus has his disciples get the donkey, he is fulfilling various scriptural prophecies.

For example, Jesus is fulfilling the prophecy in Zechariah 9:9.  There, we are told that the king will come riding on a donkey.  Therefore, Jesus needs to enter Jerusalem at this most important time on a donkey.  He cannot just walk into the city since he realizes that this is the beginning of the events that will lead to his death.  In addition, it was generally the case in those days that animals that were to be used for some sacred purpose were not used for anything else.  This is why it is important that the colt/donkey had never been ridden before.

So, Jesus is not just obtaining the colt for practical purposes.  It has a symbolic importance as well.