Allen and Greenough's New Latin Grammar, section 408

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USES OF THE ABLATIVE AS INSTRUMENTAL.

408.

Means, Instrument, Manner, and Accompaniment are denoted by the Instrmental Ablative (see §398), but some of these uses more commonly require a preposition. As they all come from one source (the old Instrumental Case) no sharp line can be drawn between them, and indeed the Romans themselves can hardly have thought of any distinction. Thus, in omnibus precibus órábant, they entreated every [kind of] prayer, the ablative, properly that of means, cannot be distinguished from that of manner.