Hi Martin,
The far eastern electronics industry realised that it costs a lot less to just manufacture one receiver that fits all, instead of having multiple assembly lines in operation. It just makes sense and you can use whatever channels you have for anything you want. So there are rarely Boat/Car/Aircraft receivers now, they are all the same, usually 6 channel as well.
I tend to use Spectrum compatible Orange RXs which you can often buy for a £5.and I've had no problems with them.
As for what would you need them for?….Well, generally you don't need all them channels, but that is not the point, but if as Kip states you want Tank steering you have that option.
I often fit an electric steam producing unit in my models, and as they run off the main battery, I can switch it off when it runs out of water which is about 20 minutes out of a 45 minute running time.
I've also had cranes working, up, down, left, right and raise and lower, there's 3 more channels right away.
What about guns, left right, up down and even fire
How about if you want sounds or lights. One friend has an Adolph Bermpol, a german rescue boat with a small operational boat on the back. He can launch and recover the small boat, drive it and steer it as well as the mother ship, all from one transmitter and two receivers.
If you only want forward and reverse, left and right that's fine but you have the option to do much more, let your imagination run riot…..It's all just fun after all.
Ron Rees.