To illustrate a simple example, when I encounter the word "claustrophobia", what I already knew is the left part "claustro-" means "small and enclosed", and I want to discover if "-phobia" has a fixed meaning of "fear". So if I have a word-list, I can search all the words that has "-phobia" and look up their meanings to confirm my guess.
Perhaps there are other reasons I need a word-list. For example, I sometimes want to discover questions like which words has the form "xxyyzz" (like bookkeeper) or which words has the part "ijk" (Dijkstra).
I've found a few word-lists but they are too small to include more complicated words like names of famous person and places, technical terms or obscure words.