At a certain point in the history of Mīmāṃsā (and, consequently, of Vedānta), the discussion of the reasons for undertaking the study of Mīmāṃsā becomes a primary topic of investigation. When did this exactly happen? The space dedicated to the topic increases gradually in the centuries, but Jaimini and Śabara don’t seem to be directly interested in it.
Nonetheless, Śabara needs to explain a related topic, namely when studying the Mīmāṃsā —before or after one’s study of the Veda. Kumārila and Prabhākara introduce the prescription to learn the Veda (svādhyāyo ‘dhyetavyaḥ, see Kataoka 2001b) and the one to teach the Veda, respectively, as the prescriptions prompting the study of the Veda and, indirectly, of its meaning. Kumārila explains that the prescription to study the Veda does not include a result which can be independently desired and that one therefore needs to insert the knowledge of its meaning as the result. Prabhākara explains that a teacher needs to know the meaning of the Veda in order to teach the Veda and that the dignity of being a teacher is something independently desirable.
The space to the topic of why studying Mīmāṃsā and which prescription promotes it increases drastically —I would say— after Śālikanātha (8th c.?). Why did this question become relevant? Perhaps because its answer was less obvious and one needed to persuade a different kind of public. A public who knew of the importance of studying the Veda, but was not immediately convinced of the importance of undertaking also a detailed study of the Mīmāṃsā exegesis. I wonder whether part of the problem is due to also to a) Śaṅkara’s statement that the Vedāntins do not need to study Mīmāṃsā and b) the fact that the Mīmāṃsā presents itself as a Vedic exegesis, but in fact looks at the Vedas from the vantage point of the Brāhmaṇas, so that an audience more interested in other parts of the Vedas might be less convinced of the usefulness of Mīmāṃsā.
Veṅkaṭanātha, though primarily a Viśiṣṭādvaita Vedāntin, dedicates the first 28 pages of his commentary on the Pūrva Mīmāṃsā Sūtra to this topic. He refutes both the Bhāṭṭa and the Prābhākara points of view. The Bhāṭṭas are wrong because the knowledge of the meaning of the Veda is not something independently desirable. The Prābhākaras are wrong because the prescription to teach is not sufficiently established and, even if it were, it would not include the knowledge of the meaning of the Veda.
Veṅkaṭanātha analyses at length all position and then concludes briskly that the study of Mīmāṃsā needs to be undertaken out of one’s desire (hence the desiderative ending in PMS 1.1.1). In order to legitimate this desire, Veṅkaṭanātha is able to show that PMS 1.1.1 (through the linguistic expression atha) shows that taking time to undertake the study of Mīmāṃsā does not violate other prescriptions and that there is a suitable time for it.
European readers may feel some sympathy with Mīmāṃsā authors, who were possibly just intellectually interested in Mīmāṃsā exegesis, but had to face external challenges and to structure their intuitions about the Mīmāṃsā being “interesting” into a consistent research project.