As Kyle said, you have just discovered semasio-phonetic group of kanji, (one of six kanji groups), which is the largest of them all - approx. 85%. Some of the phonetic radicals may play a semantic role, but not always they do. They however sure do help in reading kanji.
Some of those radicals were added throughout the history of kanji evolution, to create more complex characters. Simply, more characters were needed to express continuously growing vocabulary. Since today we have over 85 000 kanji out there, plus all those that have not been deciphered yet, you can imagine the scope of this process.
Have a go at this article, in which I write about kanji 気. This character is one of the members of the semasio-phonetic group.
Kanji: 気 | | Beyond Calligraphy