Considering go as a game of territory, blocking off is perhaps the fundamental operation once the big points have been taken. If the opponent plays from outside so as to threaten your territory, you must eventually prevent your area from being reduced. Otherwise it will eventually all disappear.
For example here is blocked (stopped as a major threat to White's territory on the right) by .
Perhaps White will feel like playing at a before , rather than be forced: that's a sound attitude. But is likely to be necessary to preserve White's area. This is rightly mentioned on the basic instinct page (point 5 there). From the point of view of strategy one may wish to exchange territories, or put more emphasis on the centre. The block is, however, a fundamental idea, for the simple reason that if White ignores , Black's next play at is very good.
If White does play tenuki, we can get this position next after and the immediate block .
Now Black has two further chances. Black can cut at a, at a later time when White cannot capture the cutting stone in a ladder. Or Black can play the clamp at b, an endgame tesuji. White would lose more territory.
This diagram shows the problem with blocking too directly. After , White still has a cutting point here. At different times White could answer at b (stepping back one line in order to block more securely), or at a (play more solidly, in order to counter-attack). These are two basic strategies for successful blocking when the direct approach fails for tactical reasons.