Chunk loaders are a workaround for the way Minecraft's game engine works.
The world is split into "chunks" of blocks. To avoid the world becoming unplayable if the player(s) explore a lot, only the world chunks immediately around a player will load and the rest of the map is unloaded and idle. This has the effect of putting all the unloaded chunks in a wierd stasis. This doesn't affect vanilla Minecraft at all. Machinery made possible by mods can be utterly wrecked by this effect though, as it's possible to have half the system in stasis and the other half loaded. If, for example you use an engine that requires cooling, and your coolant is pumped in from a distance, if the pumping station is in unloaded stasis you will very rapidly run out of coolant even though there's nothing wrong with your system!
A chunk loader keeps the area around it loaded even when no players are around. As well as preventing the bizarre issue I just mentioned, it also allows your base machinery to stay running while you're out exploring, so they're well worth having even on single-player.
The one downside, of course, is that forcing more of the world to stay loaded also boosts your server's RAM usage.