"You can define if search index files should be created during data reload, or if they should be created after the first search request of the user."
What does it really mean when referring to search for Qlik Sense? Is it referring only to smart search, or does it also mean search when you are searching for values in filter pane fields, table column searchable fields etc.
Filter panes and dimensions already represent all unique values in a field, so an additional indexing would not add much value. Indexing will however increase performance for global search.
The indexing takes time during reload, so during data modelling work it might makes sense to disable to allow for quicker iterations. While during task reload it adds value to have index composed at reload time to speed up business user performance.