There is a much more complicated reason instead/as well (which varies a bit with the situation, and becomes important if you run the rear light only from the generator), but you probably won't be interested in that.
cheers
Actually, I am interested Would you mind elaborating?
Most front lights pass on AC to the rear light, and the rear light sorts its own standlight etc out (if it does something different it will normally make it clear in the instructions). However it matters if the front light is there or not.
Generators of the hub dynamo variety produce a (no-load) voltage which is roughly proportional to the speed of rotation (some have been measured over 100V AC). However they have very much finite internal resistance and there are other tricks which are played internally which mean that (to a first approximation) the generator won't produce more than a certain amount of current, and that the speed at which that current is produced varies with the load that is driven by the generator.
The usual spec to which generators are made is the Stvzo (German) spec. This spec (IIRC) calls for generators to be able to drive into a (purely resistive) 12 ohm load and that a nominal 3W (no more and no less within defined parameters) is produced over a wide range of road speeds. Another requirement is that more than a certain percentage of 3W is produced at some low speed, (something like 80% @ 12kph springs to mind, but do look it up if you are interested). This spec has been in existence for many decades and (along with rules that said battery lights were verboten) essentially meant that for a long time all German bicycle lights were the same; a 3W generator system. Most other countries in Europe to some extent harmonised their regs with the German standard which (amongst other things) meant the original Sturmey Archer dynohub went out of production sometime around 1980.
You can tell if a generator(or other lighting component) meets Stvzo regs because it will be marked with a number " ~K***" . in recent years they have relented; some battery lights are now legal in Germany and so are nominally 1.5W generators.
Anyway if you connect a 12ohm load to a typical 3W hub dynamo it will perform to (or close to) Stvzo standards. Thing is, modern LED lights are not like 12 ohm loads; they quite often don't conduct at all until a certain threshold voltage is reached and when they do there is a relationship between voltage and current that varies with speed in a different way. The upshot of all this is that
1) you normally get a greater fraction of the nominal light output at low speeds and
2) if the LED in the headlight isn't able to use all the current produced by the generator, there will be some kind of voltage regulation built into the headlight.
3) As long as the headlight is working normally you shouldn't get much more than 6V AC down the wires that go to the rear light.
4) Provided you are happy that you are going to be riding fairly swiftly, you can wire two nominally 6V headlights in series and they will illuminate (giving about twice the usual power), just at higher speed than normal. [If you use this scheme it is important that you wire the rear light in parallel with one light, not both.]
So in essence by loading up the generator the front light exerts a strong influence on the voltage that is passed to the rear light. Without that load, the voltage would be way higher.
So read the weasel words in the rear light instructions carefully; there are three main variations
1) rear lights that make no mention of voltage regulation; if the front light fails these lights will usually blow up instantly
2) rear lights that have voltage regulation that is suitable for temporary use. These will survive a short period of 'rear light only' running but not more than that (not reliably anyway)
3) rear lights that are designed to operate in isolation, i.e. without a front light at all.
So depending on what you want to do a light of the second or third sort is probably the better choice. If not then a loose wire or something might cause the rear light to blow up, which would be a PITA.
The third sort will cope with voltages that can be about ten times higher than normal; many recent B&M designs are of this type, but not all.
hth
cheers