Do you want to show the fields to users only when they need them? Do you want to set the requirement fields according to the selected status? Or would you like to create the sophisticated configurator in your CRM?
The Dependent Fields add-on allows you to set dependencies of fields on the dropdown values. Dependent fields may not be only dropdowns, but you can work with each field type. Offers this add-on something more extra? Yes, you can also set the required fields according to the selected value in dropdown. You can use it in case when you need user to fill defined fields according to sales stage. This fields will be simply required in this stage.
You need to know the reason why the opportunity is closed lost. You can set the field Reason as required in the Closed Lost stage. You can see the settings on the picture below.
Maybe you need to display the Reason field only in the Closed Lost stage. This field is not important for other stages. You can see the settings on the picture below. Important is the invert settings, so you don’t need to select all other stages to hide the Reason field.