Currently you can set input values (eg) directly from a workflow but you cannot (eg) set the text of a text control. This is true of many other elements and properties also.
It would be good if workflows had get/set access to all properties of all elements. It’s hard for users to find and absorb the current workaround based on custom states. This is discussed in Basic question: Setting field value from a button
The workaround for the text label example above is to set a custom state of a holding input or the page to the desired value and bind the text control to the custom state’s value. This involves a seemingly unnecessary step though and I’m not sure all properties can be covered in this way. You could also use hidden inputs as the holding place for the relevant data.
You might think about moving "make changes to [a thing] an input from “Data” to “Element Actions” while you are at it.