The button causes the lens iris to close down to the aperture the shot you're composing will be fired at so, in theory, you can asses the depth of field by looking at what looks like its in focus on the focusing screen.
Well fine but on the 300D at least the focus screen is quite small and it's not easy to make out what's going on.
The reason it gets dimmer is - the iris closed down so less light gets in!!
If it doesn't get dimimer, you've a wide aperture set and the iris isn't going to close up much - if at all - either during the shot or during preview.