In the long run, why should you memorise kanji once you've memorised associated vocabulary?

To add to above, often simpler looking kanji serves as phonetic component in more complex kanji, so it’s onyomi reading will be the same as for the other ones. For example 丁 (ちょう) is used as phonetic marker is the following kanjis: 町, 庁, 頂.

If this is unclear, here is a script that adds phonetic-semantic information: