Resistant varieties offer immunity or near-immunity to many common pests and diseases, from the natural variation, breeders have selected strains that do not suffer as much from this or that problem.
Generally speaking, there Are more varieties available with resistance to various fungal and bacterial diseases (mildews, rusts, molds, and rots) than to pests; II guess because hungry pests are harder to deter.
Because they have been selected and bred for their resistance, they don't always provide the best quality in terms of cropping, flavor, sweetness, or perfume.
Nonetheless, most resistant varieties are well worth growing, especially in the vegetable garden where freedom from (or at least resistance to) any given problem is important.
The range of resistant plants and seeds is huge and includes everything from scab-free apple varieties to parsnips that are less prone to canker, leeks that get less rust, black currants and gooseberries that don't succumb to mildew, carrots that suffer less from root and lettuces that attract fewer aphids on their roots.
A word of advice, though - don't buy a variety just because it is resistant to some pest or disease unless that problem does affect your plants anyway.
Seed companies like to boast that such and such a variety is resistant or even immune to some particular disease, but you may find that this disease only affects that plant once in every dozen seats, or is only ever a problem in commercial greenhouses.
Another consideration is that although a resistant variety may be immune to a particular pest or disease now, it may not be so next year when the pest of disease has mutated.
If disease-resistant varieties aren't for you, you might like to try growing an alternative, trouble-free plant in place of one that is susceptible to disease; celeriac, for example, is much easier to grow than celery, especially if you only want it for cooking, rather than in salads, kohlrabi is much less troublesome than cabbage or turnip, while the Asian pear suffers fewer problems than other sorts.
Almost all plants may suffer attacks from common pests such as birds, rabbits, dogs, cats, kids, slugs, and snails; however, rare or unusual plants seldom suffer from as many problems as those common plants that everyone grows; there are plenty of pests and diseases to trouble roses and apple trees, for example, but few bother with zaluzianskya or mesembryanthemum.
Resistance is often just as matter of coming up with a neat solution; for example, roses with glossy, shiny leaves generally suffer less from leaf problems than those with mat leaves.
In the vegetable garden, tomatoes that are grafted onto disease-resistant root stocks can be introduced into soils that would otherwise have been too infested with disease to make cultivation possible.