People use special symbols to show how a word is pronounced because letters are pronounced differently from language to language and within languages — especially English. The two most popular systems for doing this are the IPA and diacritics (accents). Diacritics are especially popular in American dictionaries.
To illustrate, if I wanted to tell you how to pronounce the word cat, I would write [cæt] using the IPA and /căt/ using diacritics. You may ask what æ is. Well, it's a ligature (merge) of an a and an e, representing a hard a. You already knew that right? Of course, there's no chance of confusion with the Latin æ, or the Icelandic æ, both pronounced like i as in hi. Lets try another example: if I wanted to tell you how to say shhh!, I would write [ʃ] in the IPA and /š/ using diacritics. The ʃ is a long s, used in the old days to represent the letter s in the middle of words. The š would be recognized by those who speak Czech and (probably) Polish for what it is, while those who see the long s at first don't know what to think. Strike two.
Of course, this is assuming those IPA characters are displaying in your browser. Internet Explorer has a problem displaying them in certain fonts. Complain to an editor from Europe on Wikipedia about it, and they'll often tell you, "Just download Firefox [a web browser]." Well, I've got a better idea: why don't those people admit that their way of doing things is wrong?
Business drawbacksThe IPA is also expensive to print because of the extra characters that have to be typeset. It's confusing to most people. So, since common sense dictates that you tailor your product to your customer, using the IPA makes no sense most of the time. It's not easy to type, either. If I want to type ä, I can switch keyboard layouts by typing CTRL + SHIFT + 2 and then pressing CTRL + '. (You have to add a layout first in the Regional and Language Options box in the Control Panel.) There isn't an IPA layout, though. So, I have to open MS Word → Insert → Character, then scroll down to IPA.
The IPA is made up of 107 symbols. Only a fraction of these are ever used, and many are identical. For example, compare [a] and [α] (diacritic symbol, ä). Both are pronounced like the a in father, and it's my opinion that they actually are identical. However, some linguists will quickly correct you to say that the tongue drops forward with [a], while it deflates backward with [α]. Now, don't punch him -- it's his job. But, still: who cares whether the tongue falls forward or backward? The answer is, no one except linguists. So, why are these symbols used in mainstream British dictionaries? I have no clue, and people from England generally don't, either. But, don't try to use a diacritic transcription on Wikipedia, unless you enjoy getting your edits reverted.
I'm not saying that diacritical systems in use today are perfect. For example /ī/ as in die is really two sounds: /ä, i/ (i like the e in eel). Likewise, some, like The American Heritage Dictionary, dropped some diacritics (e.g., š in favor of sh). Still, then why not perfect it? Even without reform, it would still be better than the IPA.
For a sample chart of characters using diacritics, click here. For a chart of IPA characters, click here and scroll down.