A botamochi with taro in it. Glutinous rice was expensive and scarce, so ordinary table rice was used instead and taro was worked into it, and the taro supplies enough stickiness on its own that the result behaves very much like something made from glutinous rice.

The description given is direct about the reasoning. This is a way of stretching rice, invented by farming households who could not afford to be casual with it.

Other prefectures have things they call imobota as well. Those use potato, or sweet potato, rather than this. Nara uses taro, and the reason offered is proximity: this is paddy country, taro grew well here, and taro was what was to hand.

It is also, apparently, easy on the stomach. You can eat a lot of it without discomfort, which is a real advantage in something intended to fill people up. And it acquired a ceremonial life. Eating mochi on the day of the boar in November, when the harvest is finished, is said to ward off illness, and at the equinox and the autumn festivals imobota is placed on graves and household altars. A method for using less rice than you otherwise would have ended up as the thing placed in front of the dead.