Removing long spindly tops so plants will fill out from the sides and lower branches.

Deadheading is the removal of spent or nearly spent flowers so that the plant can use those energy reserves making more plant body rather than seeds (or fruit).

Pinching is the removal of the apical meristem so that axillary buds will break out of dormancy and grow.

Roses (modern not the antique ones) are a good example of where to use dead-heading. Most people outside of the this forum grow roses only for flowers. Removing the spent flowers prevents the rose from spending energy on making the hips (rose fruit) and may cause them to rebloom. If you want to collect seeds or have fruit, forget the dead-heading!

I'm sure you can search in some of the perennial forums for whole dissertations on dead-heading.

FataMorgana

A plant's reason for living is to reproduce through flowering and setting seed. If the cycle is interrupted by removing the flower before the seed head develops, you get more branches, more flowers (as in the case of roses) and more leaves (which is the useful part of most herbs). Dead-heading is simply removing the spent flower for the reasons rhizo stated. It also makes the plant look better.
I enjoy dead-heading because I can see what is going on with the plant, how the shape can be improved, and collecting all those aromatic leaves/petals is a pleasure!

I like to deadhead, too! I de-capitated about a zillion spent pansy blossoms this morning. That little bit of activity will result in a nice flush of new flower buds.

That's the most important benefit of deadheading, and I didn't even mention it! Continued bloom!

The local Lowes was selling a big batch of potted pansys and all the dead flowers were clipped off. The pots went for 50 cents each.