Groundcovers are low-lying plants, usually chosen with aesthetic considerations in mind and requiring minimal maintenance. If they are projected to cover large expanses of ground on the landscape, the initial cost will be much greater than for sowing grass seed, but groundcovers may save you money in the long run, as expenses such as tune-ups for lawn mowers will be eliminated.
"Groundcovers" and "cover crops" are different groups of plants, despite some overlap. Cover crops are sometimes used as "living mulches," in which function they serve a role not unlike groundcovers. But cover crops are often tilled into the soil not long after being planted, simply to supply the soil with amendments, whereas groundcovers may inhabit a stretch of land for many years.
Groundcovers can be saviors for homeowners who are stuck with having to mow steep slopes (or who are simply unable to get grass to grow in such areas). If you are plagued by such a hillside and it is in a sunny area, my article about 'Blue Rug' juniper groundcovers may be the answer to your problems.