Although DNA is an excellent medium for the storage of information, the very characteristic that makes it so stable and inherently self-correcting - being double-stranded - also makes it unwieldy for using that genetic information to make cell components. Since the informational parts of the molecule (the nitrogenous bases) are locked inside the ladder, reading it requires the energetically expensive task of breaking all the hydrogen bonds holding the two strands together. To do so for every single copy of each protein needed by the cell would not only take a lot of energy, but a lot of time. Instead, there must be a mechanism to take the information from DNA once (or a few times), and then make many copies of a protein from that single piece of information. That mechanism is transcription.
Thumbnail: Simplified diagram of mRNA synthesis and processing. Image used with permission (CC BY 3.0 - unported ; Kelvinsong).