Dashboards include a concept of 'presentation variables', which can be created and then referred to in filter formulas. A presentation variable is great for accepting user input, however sometimes we also have a need for calculated variables which do not depend on user input, or which themselves refer to presentation variables where the user has made some input.
In theory, the calculation formula could just be incorporated into the formula for the filter, but this has a major drawback. If the filter is running on a table with 10 million rows, then Incorta will calculate that formula 10 million times to find the value of the variable. This can sometimes cause unacceptably slow dashboards.
In the instance where the calculation is not dependent on the record, and will always return the same result, we should be able to add this as a variable which can be calculated once, and then refer to that in our formula filters