You can use dimmer/switch packs if you want to be able to control power to the lights remotely. Many dimmer packs also allow you to turn them into switch packs, wherein the pack won't dim the lights based on DMX value, but turn them on above 127, and off below that.
So, when do you need them? For example, in a club with your lights permanently installed on the ceiling, you want to be able to remotely control power to them. Sure, you could put them all on a breaker, but it's much better to have complete 100% control over power to each light from the lighting station.
On the contrary, for a wedding as another example, a dimmer/switch pack would be a bit overkill, unless you're using incandescent par cans and you want them to flash or be "dimmable". The bottom line is, you don't NEED a dimmer pack for certain situations at all but in others it is quite advisable. It's about convenience, really.
- Anytime you're dealing with incandescent par cans or any fixture that is "on or off" only, you probably need one. Unless, of course, you want those lights to remain on until you physically unplug it or otherwise somehow cut power to it.
- If you want to be able to remotely control power to individual lights, you need one, and if any of the lights plugged into it have circuitry/computer/etc, it then needs to be set to operate as a switch pack and not a dimmer pack.
- If you don't care and you want them to be powered on at all times, you don't need one at all.
In my club which is relatively small, I use several 4-channel switch packs just to control power to the lighting system. The Elation (DP-415 I think) switch packs we use can be dimmer packs as well, but I used the dip switches to make them switch packs only so that I don't damage lights by under-voltage. You don't want ever to "dim" a light that has any form of circuitry sensitive to voltage. For fixtures with any kind of circuitry, you want them to be on or off only.
Hope this helps.