When/wnhy does coolant/anti-freeze get used?

[tomw2000]

I'm a bit of a mechanical divvy as it were and was wondering if anyone might explain when engine coolant/antifreeze gets used/consumed?

What I mean is, sometimes a car hardly seems to use any, but sometimes it needs topping up. Why? What causes it to be consumed(or not)?

Wife has a 1yr old RRS on 27k miles and I had to top it up the other day - and it got me thinking, why.

Many thanks in advance.

[Defcon5]

It is a sealed system, so should never need topping up really. It its being used its leaking from somewhere

[tomw2000]

Thanks for that. Car's in to the main dealer next week anyway (and not being used before then) so I'll get them to look at it.