Hides the truss rod, and provides a harder surface that is not as prone to string wear.

Fretboards, or fingerboards in this case since it's fretless, are wear items. They need to be harder than usual neck woods like maple are to wear out slower, and it's nice to be able to replace them after they're worn out.

Violin strings are metal wrapped, but pretty soft compared to steel banjo strings. Even in that case they wear out and have to be replaced.

On some electric guitars, like some fender teles, etc, maple fretboards as a single piece with the neck are not uncommon, even with steel strings. That's because the frets take the brunt of the wear. But without frets I don't think they'd last very long.

If your banjo is using nylon strings you can probably get away without it. You can also just be cool with the worn out look and go with it. On a fretless it's not going to buzz if the fingerboard wears like you can have with worn frets.

The last factor is that banjo necks are long and skinny. They usually need a truss rod for reinforcement against warping and bending. The fingerboard/fretboard hides the truss rod. A lot of early banjos used laminated hardwoods (visible on the back of the necks) before truss rods were invented. This works too, nothing to hide.

For a fretless, your main consideration is wear from playing. Fingernails make dents and gouges, wound strings can be abrasive.

Mahogany is quite soft, so if that's your neck wood I'd consider adding a fretboard. Maple is usually hard enough but shows dirty marks fast - either give it a Fender electric style heavy finish or use a separate maple fingerboard dyed black. Oak is hard enough, but you'd want to fill the large pores to make it smoother.

If you could find some 1/16 (1.5 mm) rosewood or ebony veneer, that would work well and be the right colour and easy to do.

You could strengthen it with an epoxy resin. It only needs to be a thin layer if that’s the look you’re after.