Here's two reasons you many want to drill a tank. First would be if you'd like to run a sump. If you run a sump, you need a way to drain the water to the sump. The best way to accomplish is to have one or two holes in the tank that allows water to drain out. The other way that doesn't require drilling is to use an overflow box. Overflow boxes can work well, but can also cause your tank to overflow is the siphon would break.
The other to drill is to run a closed loop. A closed loop is just a pump hooked up externally that sucks water out of your tank and pumps it right back in. Its just for circulation. The other way to increase circulation is to use power heads. Small pumps that just stick inside the aquarium that push water around.
If your just starting out and are getting a new tank, I would recommend getting a 'Reef Ready' tank, meaning it's already drilled for a drain. If you already have a tank, you can just stick with what you have and tweak it later.
I don't have any pictures, but if you search on sumps, drains, closed loops, powerheads, reef ready on this site, you'll be reading for weeks.
Best of luck,
Ron
