Well, the most obvious reason is to make more money.
The downside of playing 1 poker table is that you are missing out the major profit opportunities.
It's a fact, experienced players will make more money by playing more tables.

The multi-table game means more action, but also less time for decision making.
The multi-table game might not be for everyone.

What could be the downside of multi-tabling?
More tables means less focus on each table.
You could miss small details from the tables.

The downside of playing to many tables could lead you to play less well on each table.
Some players feel that multi-tabling becomes a mechanical game and therefore don’t like it, it’s actually a matter of taste.
When you feel that the multi-table game affect your results it is time to take a break or simply stop. It's very important to closely monitor your results.  

Conclusion: Multi-tabling does not fit for everyone.

If you believe this game is for you, then start out with 2 tables, if it works out fine you can ad more. Some players are comfortable by playing 4, 8 or perhaps 12 tables.
Playing 8 tables or more are for a limited number of players. The more tables you play the quicker you have to analyze the situation and make a fast decision in short time frame. One other important thing is that you must be able to control your emotions, losing out on a few games can affect your emotions in a bad way. The bottom line is if you are able to handle the heat, multi-tabling is the best way to maximize profits. No matter if it’s 1 or 10 tables you prefer playing I wish you luck. You can find more helpful poker articles here.