You will need to know if your soil is acid or alkaline (or neutral). An Acid Soil does not have much - if any - lime in the soil. An Alkaline Soil does have lime in it - to varying degrees. A Neutral Soil; well, it has lime in it, but not enough to class it as an alkaline soil.
You can get a cheap basic testing kit at most garden centres to tell if your soil is acid or alkaline - and to what degree. (You can also do a basic test by drying a teaspoonful of soil, and then sprinkle some vinegar on it. If it bubbles, it will probably have lime in it. If is doesn't it will probably be acid or neutral!)
Most plant require lime in the soil to live and thrive. Rhododendrons, Camellias, Ericas and a few others do not. In fact the presence of lime in the soil will make them quite ill - probably terminal
Lime encourages soil life, for the bacteria that sorts out your organic matter into Nitrogen, are quite lethargic in acid soils. It improves the 'tilth' (crumb structure' of heavy soils such as clay soils. A really sticky clay soil can be put right quite dramatically with a dressing of lime. The lime coaxes the individual clay particles to form 'groupings, allowing moisture to drain, and plant roots the freedom to roam.
Lime can act as a deterrent for some pests - slugs and leather-jackets are not keen on lime. It will also act as a preventative for club-root in Brassicas.
An alkaline the soil is good for earthworms. Most soils will benefit with a high earthworm population. As well as helping to break down raw organic matter, they make a network of drainage channels in the soil - great for heavy clay soils.