Mulch - how important is it in the garden?

Living in the tropics, I find that the soil must be covered at all times, either with plants or mulch. The pine bark mulch at our local supermarket was on special, so I rushed in to get 14 bags. I tend to switch what type of mulch I use in the garden over time. Last year I used hay mulch, and the time before it was peanut husks. Of course, all the time I also have lots of leaves falling from my lychee tree. I do pick up a lot of those leaves for the compost, but plenty lie under tree slowly forming leaf mold. They can draw nitrogen from the soil as they break down, but I find a sprinkle of home made compost on the leaves every now and then seems to fix that problem. The first area I added the mulch was the little bed right outside the bedroom window. I had weeded that bed the day before, and removed the mondo grass edging as it was getting too invasive. I think the ground orchids will grow well in that area now. Those ar...