Mulching provides a number of benefits to your garden beds. Firstly it moderates soil temperature, keeping it cooler in summer by shading it, protecting it from the harsh glare of the sun. It also keeps soil warmer in winter, acting like a blanket on those chilly cloudless nights. It reduces soil evaporation by between 30 to 50% which greatly reduces watering requirements. This is because mulch on top of soil reduces it's exposure to the doubly drying combination of wind and sun.
Another benefit of mulching your garden beds is a greatly reduced incidence of weeds in them. As long as the mulch is applied in sufficent quantity, it blocks light from reaching the weeds which they need to grow. In addition to this, any new weed seeds entering the bed, (e.g. dispersed by the wind blowing through your neighbour's weed infested garden) land on what is essentially bark or woodchip, not soil and so they can't germinate. If the actions of rain or reticulation eventually wash those seeds through the bark/woodchip layer to the soil beneath and germination does occur, those tiny weeds should expire before they reach light, which they need for further growth.
Yet another benefit is that mulch, as it is slowly broken down by soil bacteria, fungi and other organisms, releases nutrients into the soil making them available to your plants. Depending on the types of plants and mulches involved, a properly composted and mulched garden bed will rarely if ever need any artificial fertiliser inputs, which again goes toward recouping the initial cost of purchasing the mulch.
The next post will deal with choosing the best type of mulch for your garden.