The currently to let toggle can be used to mark the letting unit as available for rent. This has useful features within Tenancy Manager for example, the enquiries list will only show currently to let letting units by default.
A letting unit can also be marked as "currently to let" from the actions button next to every record in the list of letting units. This saves having to view then edit the letting unit to make this change.
If a letting unit is already marked as currently to let, it can be switched of by being marked as under offer/let from within the same actions button.