There are a number of parts to this answer.
First, the catcher is the only player on the field who sees the whole field. Therefore, he's the only one who can adjust the plans, at any time, in response to what's in front of him.
Second, the pitcher can and usually does take these signs as suggestions. A pitcher can change this, and hopefully, does so after consulting with the catcher, either by a conference on the mound or by signals back to the catcher. If you've ever noticed a pitcher moving his gloved hand across his abdomen or his thighs, you've seen a pticher changing the plan by adding to or subtracting from the numeric sign given by the pitcher. Of course, if the pitcher doesn't let the catcher know this, he can "cross up" his catcher, and that really means that he's crossing up himself.
The catcher will also give signals that indicate that the defense should shift, or be alert for this or that. You see that when a catchers stands and moves his arm around in front of his chest protector.
As well, the manager can signal things from the dugout, either for a fielder or for the catcher to signal to the pitcher.
Some pitchers get so involved in their game that they don't always realize that the situation overall has changed. In such cases, the catcher is there to keep the pitcher focused, bit not so focused that nothing else exists.
Some ptichers simply don't think on the mound, while others tend to think too much. Both of them need their catcher to guide them through the game.
Yes, part of a catcher's job is to handle his pitchers. And the best of them think in just those terms - the team's pitchers are HIS pitchers. But part of a catcher's job is to handle the rest of the team, to read and handle the batters and to handle the plate umpire. (My apologies to Ryan R for that one, if I've offended you with it, but you do know that it's the truth.)
Then, of course, there's Casey Stengel on the need for a catcher. When the Mets were granted a franchise and going into their first draft, he was asked which player the Mets were planning to draft first. He responded that they would take a catcher first, because without a catcher, you have a lot of passed balls.
Dude was wrong when he generalized about young catchers. There are catchers who are there because they can hit (MIke Piazza is one), there are catchers who are there because they can throw (Pidge Rodgriquez, Benito Santiago come to mind) and there are pitcher's catchers, catchers who get into their pitchers' heads and work with them.
Don Sutton had nothing but the best to say about Charlie O'Brien. He tells a story about going to Oakland, and working with O'Brien when O'Brien was a raw rookie. In his first start with O'Brien behind the plate, in one of O'Brien's first starts as well, Sutton called his own game for the first couple of innings. Before the 3rd inning started, O'Brien went to Sutton and said that he felt he knew what Sutton was trying to do on the mound, and asked if Sutton would allow him to take over calling the game. Sutton said OK, figuring he could always add or subract, or if necessary, return to calling his own game. Sutton went seven innings in that start, and never shook off O'Brien.
This is what a pitcher's catcher, no matter how much major league experience he may or may not have, is capable of doing.