I used to have an 8-string Uke - the G and C strings were each tuned an octave apart, i.e., G4 and G3 (I believe) and same for my C strings.
I used it primarily for strumming, as plucking with an 8-string Uke is somewhat hard. It sounded very nice actually and I wish I still had it - my brother who resides in south San Francisco has it so yeah...
But...the point of it is to produce a different sound basically - like what's the point of a low/high G? Ya know? Yeah.
6 and 8 string 'ukuleles are used mostly for strumming to provide a fuller, rounder and deeper sound. The 8 string has four courses that are doubled. The 1st and 2nd are in unison (AA & EE) while the 3rd and 4th are octaved (Cc & Gg). In the case of a 6 string, the 1st and 3rd courses are octaved (Aa and Cc). Both are played the same way you would a 4 stringer. You can also finger pick on both of them, but more preferably on the 8.
I believe the extra strings are for more depth of sound - they are an octave apart for each doubled string. I believe you can do tremolo, like how they typically play a mandolin. I don't know if it's traditionally used for tremolo, but if I had one that's how I'd play it.