Formulas currently only accept numeric field types as input variables. This limits their capabilities as numbers do not always usefully reflect descriptive data.
For example, if you had SLAs for support tickets where enterprise customers received faster first responses, you could not use a formula field to do something like:
First Reply By =IF(Customer="Enterprise", Created + 1 day, Created + 5 days)
I get that you could use rules to basically replicate a formula field’s process, or use a rule to update a numeric field when a qualitative field changes and then use a formula field, but these both feel like poor design choices.
A formula describes what a value is - a rule describes what should happen when something changes. If a formula’s output can be derived logically/deterministically from its inputs, why use event-driven rules to maintain derived state?
Forcing users to remember numeric values for qualitative data adds unnecessary mental load. Obfuscating that by using rules (differently) to set a numeric field when a qualitative field is set adds unnecessary complexity to your data model.