Why does formula fields only allow numbers?

Why does formula fields only allow numbers?

Hello all. I am trying to create a dynamic dropdown. The entries of subform a should autopopulate a field in subform b. If this is possible, then can I then make a dynamic dropdown of which I can click on said entries in subform b? Thanks for any input


This would be quite easy to achieve with a formula field, but it only allows for numbers. Why?