Keeps your teeth safe, pant guard, chain guide.
Recommended,
A good go might be a 36-34 mid, 26-22 granny, outer mounted bashguard.
34 and 22 for hauling a heavy bike uphill. 36-24 if your o.k with weight and want to speed up. Nice thing is chainrings are easy to swap out.
If I went 'mountain climbing' I just swap my 38 for a 34.
They cost 'buck a tooth' or whatever.
Get a chain tensioner if you try a single ring setup.
>jef.