I'm aware the two kanji are often pronounced the same, but why does one contain the other in it?
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That is called 形声. About 90% of all kanjis are created in this way. In this case, the left side
In Middle Chinese and a reconstruction of Old Chinese, 吾 and 五 have the following readings (Baxter):
In modern Mandarin, 語 (yu3) and 五 (wu3) have diverged in pronunciation, but according to reconstructions they were pronounced similarly in antiquity.
There are other cases of 形声 where the phonetic does not apparently match the pronunciation -- some of these are because of this divergence.