There may come a case, as in the previous tutorial (CakePHP 3 Tutorial 18: Autocomplete) that you want to display one thing in a form field, yet save a different value. In the previous case, you select a user's name from an autocomplete list, yet the user's id is the value you want to save in the database.
In the previous two tutorials we discussed adding associated records. But what if you want to select a record from an associated table where there are thousands of records? Autocomplete can help with this issue, but it requires a combination of parts to work.