the nail almost
They have a swept magazine, so you can rotate them a bit in the 90 degree "space" as well...
Yup they are to produce the ignition spark. Yes they are rechargeable, but even rechragables only have a limited life before they no longer hold enough charge and need replacing.
you need a small
You can usually pick up one for a bit less on ebay. A £100 compressor will do the job (and has other uses)
The nailer or the compressor?
Well normally I have the compressor in the garage (which is adjacent to my workshop). Typical jobs in the garage are tyre inflation (car, bike, mower, anything else!), and driving an impact wrench occasionally. In the workshop, the most common uses are with a blow gun for dusting, cleaning etc (and inflating inflatable things!), and running a small 18 gauge brad nailer, which I use for a variety of furniture making and restoring projects. For example fixing T&G backs to things like:
For other jobs like resurrecting the remains of a wooden shed that we acquired when we moved in, a long airline reached it (nailed all the shiplap planking back on with a framing nailer (£90 on ebay) and 2" nails.
For jobs away from base, then its not actually that difficult to shift a small compressor if required (certainly no worse than a sliding mitre saw and workstand for example - and those are quite handy for studwork walls as well!)
;-)
Typically air tools need a bit of lubrication. In the case of a nailer, then you can do this by putting a couple of drops of oil down its air inlet after each use (or every so often if under heavy use). Alternatively you can use an inline oiler. This adds a very small amount of oil vapour to the air stream. These come in the form of small units for attaching to an single tool: