Why "Enable word completion" and "Collect words" settings are separated?

Solution 1:

The two options are not synonyms:

  1. Enable Word Completion : will do completion while you type
  2. Collect Words : LibreOffice will record every word you type and add it to its database of words.

The first option without the second, will use the existing database but will not add new words to it.

The second option without the first, will collect words and add them to the database. For the updated database to be used, you will need to enable the first option.