It might be to have fun, solve a problem, to gain support, to arouse emotion, to generate interest or heighten awareness, etc.
Most speech acts have more than one purpose.
Listen to a casual conversation at a party sometime and try to identify the shifting purposes. The structure of a speech act can take many forms. A story is in chronological or narrative form, for instance. Textbooks such as ours are often in topical form. There is almost always a claim, and the development of an idea.