Essentially, textual form refers to the physical appearance and structure of a text, including the way it visually affects readers, appeals to the readers' senses and elicits an emotional response. Examples include:
The choice of textual form is an important decision for any composer, and you must assess why this decision has been made, which would typically involve a closer examination of the text in question.
As far as I'm aware, purpose involves the author's intended reason for producing a certain text. For instance, texts may be aimed at entertaining an audience, while others may convey and support a particular opinion and/or ideology. The former may be manifest as a play whereas the latter may be manifest as a speech. Based on this, one could infer that textual form is a medium through which an author can achieve purpose. Elements within textual form that may assist in doing so include literary and structural devices appropriate to the respective forms. Depending on the type of text, examples of such elements can include: