I do not know why this question never occured to me before, but why do these spells require pieces of an animal to enact a transformation? I can't think of any other spells that require anything like that, and there is no indication that shape bonuses for those materials help with the spell. Am I missing something obvious?
That's so the spell can be ended early, before the duration has run out.
See the Muto Corpus guidelines for a more detailed explanation.
Well, now I feel foolish. I must have read that at some point and forgotten about it.
The only shapechanging my characters have tended to do involves a heartbeast, so I didn't have a lot of truck with those spells. Thank you for the response.
The real reason, in core, is probably that it is a legacy spells, and older editions based spell design on flavour more than guidelines.
Erik's explanation is found in the Mercere chapter of HoH, where these well-known spells are cast as a remnant of the much more complete and flexible mutantum magic. It is unlikely that the authors of core had thought of this at the time, so the explanation is post hoc.
And this, too, is correct.
In game I have ascribed it to experimentation on Potent Magic spells, with the magic theory requirements dropped for those spells.