You don't need to use speedlights or portable flash guns, but they can be useful in certain circumstances, such as when you want to include a person in your nightscape image. Personally, I prefer to use the manual, lightpainting method with a torch as it gives greater artistic flexibility and versatility for modelling the subject.
The key factor to remember when using speedlights is that you do not need anything sophisticated or expensive. A cheap speedlight will cost anything from £35 to £55, and a set of remote flash triggers will cost about £25. You do not need any TTL communication with the camera, as you are going to use the speedlight in manual mode. This makes this type of light painting very affordable and reasonably flexible. An inexpensive, table-top tripod is also useful to sit the speedlight on, especially when out in the field.
An important thing to bear in mind with speedlights is that their flash output is designed to be daylight balanced. In normal conditions, this is fine, but at night, if you have set the White Balance on your camera to anything cooler than a daylight temperature (5000-5500), then the flash output is also going to be cooled down. This is important if you are taking night shots including people's faces.
This is simple to rectify at the editing stage, but I prefer to get it right at the capture stage. The speedlight / flashgun may come with colour correction gels, and if it does, there will almost certainly be an orange covered gel. This will be a CTO gel, and is the type used in theatres and TV studios. These gels "warm up" the output from the speedlight and make it less harsh. In our case, with a cool White Balance set on the camera, they will restore a natural, daylight balance to the light they provide. This can contrast very effectively with a nightscape image where other elements of the composition have been lit by a torch, for example.
Balancing the bright output from a speedlight with the ambient night conditions, plus any additional lightpainting you have decided to do is a matter of trial and error, but very simple. In terms of the speedlight output, just set the flash sync mode on your camera to Rear Curtain Sync (refer to your camera's instruction manual), and set your flash (in manual mode) to a half or quarter output (to start with). The rear curtain sync will ensure that the flash will only fire at the very end of the exposure. If your exposure is 13 seconds, the flash will only fire at the very end of that thirteenth second. This ensures that the sensor records all the details you want to as far as the ambient light is concerned, and then the flash output is added at the very last moment. If the flash output is too dim, or too bright for your taste, simply adjust the output accordingly. As it is only a very brief "pop" of flash that you need, start by setting the speed light's output to 1/4 power, and be prepared to adjust +/- as required.
Adding speedlight flash for lighting is something I don't do very often. It is quite a labour-intensive task to perform, especially if you have more than one speedlight to set up, and it can require additional lighting stands / tripods to mount the speedlights on. This is then additional weight in your pack, and additional strain on your back!! However, you may find that you prefer this daylight balanced lighting over torchlight. Using multiple speed lights certainly adds a creative dimension to your lighting setups if you start taking night shots at all seriously. They also have the advantage of being predictable and repeatable. Again, it's up to you. There is no right or wrong.