The elder Seneca mentions several reasons why he came to write about declamations and declaimers. First, he will have us believe that the theme of his work was suggested by his sons:

This request from his sons is perhaps fictional or semi-fictional. To claim that one was writing at the request of some person was, like the epistolary greeting, a standard convention among ancient writers of prefaces to works whose utility needed to be emphasized. To make out that one's purpose in writing was to give information to some member of one's family had been a cliché in classical didactic writing ever since Hesiod had addressed the Works and days to his brother. To take two Roman instances, Cato in, for example, the dictum quoted by Seneca in Contr. I pr. 9: orator est, Marce fili, vir bonus dicendi peritus, and Cicero in his Partitiones oratoriae, had addressed rhetorical precepts to their sons, the latter in a baby-simple catechistic fashion which his rebellious offspring can hardly have appreciated. Admittedly Cicero's son was only thirteen when it was written.