They are actually many different types of cadences, each serving different purposes. It's easy to show if you think of music as a sentence. A phrase of music is a sentence.

A authentic cadence would be like a period at the end of a sentence. But they are two different kinds of authentic cadences, one being perfect the other imperfect. The difference is a perfect cadence is V-I (as discussed before) with the both chords being in root position and the melody finishes on the tonic note. The form of a authentic cadence is used mostly at the end of a musical idea. Authentic cadences have the strongest sense of finality, and for this reason they are the cadences we here at the end of a piece of music.

An imperfect cadence is sort of like the perfect one but one of the two chords (either V or I) will be played in inversion, and the melody line doesn't have to end on the tonic note (even better not to). Because a imperfect cadence has the effect making a piece of music only sound half finished and half wanting to continue. In song form it is usually played at the conjuncture of a two part (like verse to chorus). Try this cadence to end a piece of music in one key, before going to a different phrase of music in a different key. Note that for pyschological purposes, you will need to return to the original key and theme before you end the piece.

Then there are half cadences, if the authentic (P or ImP) cadence is a period, the half cadence is like a semi-colon. Half cadences are played as I (or anything) to V (opposite of authentic). This has a strong sense of unresolve and music to continue, and are usually played at turnarounds. Notice how almost all turnarounds end on V, or a sub or somewhat form of it.

Then there are plagal cadences (perfect, imperfect, and half). Plagal cadences are the same as authentics except V is exchanged for IV, so a perfect plagal cadence would be IV-I. Know as the A-men cadence, sung at the end of most hymns.