Propagating from seed
Growing from seed lets you choose the precise colours you need, which may be unavailable as bought plants, and also allows you to plan stunning displays by precisely matching or contrasting colours and forms. Many unusual and rare plants are only available from seed, e.g. from specialist societies such as the Alpine Garden Society and the Hardy Plant Society.

Sowing seed is also the best way to obtain large numbers of plants, though more work is involved - you will need to sow early, prick out, and grow on until planting-out time.

Not all seed is sown in spring. Many alpine and perennial plants need sowing in autumn as they require exposure to cold temperatures (and possibly rain) to remove inbuilt germination inhibitors. Some seed has a very short life and must be sown as soon as it's collected from the plant.

Cuttings
Unlike seed, cuttings always result in genetically identical plants - clones, in other words. This technique is useful to multiply particular plants. For example, you may have an unknown rose with an attractive fragrance - if you sow its seeds they would probably all come up distinct from the original, but cuttings will result in exact copies.