what's it for and is it necessary ?

Most common day filters have three "stages" to them.

Physical - removing debris by way of filter media (floss, cotton, etc...)

Biological - good bacteria in your filter eating up bad things such as ammonia and nitrates

Chemical - this is usually active charcoal.

The charcoal absorbs other carbon-based chemicals from the water as it passes through. The charcoal is "activated" by oxygen to create several pores between the carbon bonds giving an ounce of charcoal a huge surface area for more absorption capabilities.

This charcoal needs to be replaced periodically as once the charcoal has absorbed so much it no longer is able to continue it's job.

Most people use charcoal to absorb organic items from their water that create foul smells or to remove certain pigmentation from their water such as tanins that create a tea-like color.

__________________
I like big fish tanks and I can not lie. You other brothers can't deny.

...unless you use live plants as well. In which case, you will not want to use carbon in your filter as it actually robs the water of said carbon compounds that the plants would otherwise use.

The overreaching purpose of a filter is to provide a home for the bulk of the nitrifying bacteria you need for your aquarium's nitrogen cycle. The constant flow of water through the filter promotes much more bacteria interaction than the rest of the tank, and some filters even incorporate biowheels which function the same as bio balls in a wet/dry sump (the best filtration out there). Filtration using nitrifying bacteria is commonly called "biological filtration", and it's main purpose is to filter out ammonia and nitrite from the water, with nitrate as the by-product.

__________________
Officially fishless until after the move....tune in later for details!

Ewww...... glad you commented on this..... I've got all live plants in my tank, guess I need to take that carbon sack out of my second stage and replace that space with another foam sleeve or more ceramic rings then.....

Most definitely. The used carbon makes good house plant fertilizer, BTW. I do one foam pad and 2 bags of bio media (ceramic stuff) in my filter. The ceramic stuff just provides more surface area for the bacteria to set up shop. And you should never ever have to change the bio media.

__________________
Officially fishless until after the move....tune in later for details!

To me, the one and only use of charcoal is to remove medications, and since a healthy tank rarely needs medication, it's an unnecessary expense. I have a couple of bags lying around from filters I've bought, but it's not something I generally use.