When I first started brewing I always racked to secondary fermentation. Many have said that it is not necessary, and now I often just keep it all in the primary carboy throughout the fermentation process. I keep it there for 3 week and then rack to the bottling bucket and bottle.
Before I go further, check out this video, which does a nice job of walking through the racking process, because there are times when racking may be the right thing to do.
Here are the three reasons that I would use a secondary fermentation vessel.
1. If the beer will be aged in the carboy before bottling. A barleywine, for example, will be best after aging. Also, the addition of dry hops or oak chips will work well in secondary.
2. If I plan to "clean" and cultivate the yeast for another batch. Racking the beer off will allow access to the yeast bed at the bottom of the carboy for this purpose.
3. If another batch of wort is ready to go and dump into the original carboy on top of the existing yeast bed. It's been expressed that this works well. I've not done it, and most of the time it doesn't fit in with my brewing schedule and practices. But, this method will present the new wort with plenty of yeast.
Some downsides to racking into a secondary vessel are as follows.
1. Increased potential for oxygen exposure. The only time oxygen is a good thing is just before pitching the yeast.
2. Increased potential for infection. This risk can be considerably low with proper sanitation procedures.
To use a secondary or not is a topic talked about frequently in online forums. In a nutshell, keeping the beer in primary only will work fine. Unless there is a purposeful intention, forgo the use of a secondary.