When enabled, the allow other option will allow the user to select 'Other' at the bottom of the choice field. When selected a text box will appear allowing the user to manually enter in their selection for the choice field.
When this is used, the other option that is entered is not stored to the existing choice list and will not be presented as a choice option the next time that choice field is accessed.
This option is designed to capture outliers or options that the app designer may have not thought about or overlooked. If there is a consist value being manually entered through this option then it may be a good idea to add an option to the choice field or choice list.