Hi Sweetcorn, Lime is used to increased a soils Alkaline content to suit what you grow. Generally vegetables require a "PH" of around 6 or 7 to grow well with cabbage family memebers needing a little higher at about 7 or 8.

PH is expressed as a number from 0 to 14 with 7 being 'neutral'. 180gms of hydrated lime / sq m. will raise the PH by 0.5. its best to raise the PH to the required level over a number of years rather than all at once. Lime should be put on the soil in the Autumn on rough dug soil and left to stand til spring.

Dont get too hung up on this subject though as most veg will give decent results in most soils.

If your raised beds are 'boxed in' and contain plenty of organic matter I find no reason why they should have a tendancy to dry out. The fact that one shouldnt walk on the beds except for winter digging allows the soil to have greater penetrable depth so the roots can go deep. Thats why generally speaking, plants can be spaced much closer together using this method give higher yields per sq. m.