When we re done, we mulch between the beds to conserve moisture and discourage weeds. We went to that system for three reasons. First, to get the roots into looser soil and make harvest easier on people and plants, both. Lastly, by building the bed from the surrounding topsoil, you increase the feeding zone of the plants roots by thickening the layer of fertile topsoil above the infertile, inert dirt beneath. This additional nutrient-rich layer allows the roots to get more out of the soil and their increased strength allows them to resist pests and weather extremes better.
Water also percolates through it better to accumulate on the hard pan, which is now further below.
Even if you live in one of those areas that gets copious rainfall, even a light mulch is usually more beneficial than no mulch, if for no other reason than to keep rainfall from splashing dirt up on the leaves and then having it wash down into their bases and, possibly, contaminate the bulb with some kind of undesirable soil-borne bacteria or fungal spores. If you re in a dry area, a thick mulch is essential if you want good results. By alternately irrigating or removing mulch, you can control the soil moisture to maximize bulb size and health. Using organic mulch has an additional benefit - you re fertilizing the soil for next year. We found that when we put several inches of mulch on the entire growing area that our harvest was better all the way around and the soil was less weedy and more fertile and softer for the next crop.