Beast Transformation spells elevated the ranker's body stats as well as heightened their mana storage for the time till their transformation was active. It also provided them with a few beastly features that could be visible or invisible.
Layla had predicted that Rodrick's demon beast was somewhere in the Aranger city because of how his other transcendent-grade spell worked. And she was right. But only partially.
Rodrick had used the Beast Transformation spell to keep Aqua close to him when he had entered the city. This was another benefit of having a Beast Transformation spell that the demon beast tamers had high use of.
The Beast Transformation spell required one to have a spatial runic tattoo first. This would enable them to create an isolated space near their heart through the use of spatial magic. Only demon beast tamers who had a compatible mana signature with their beast could do something like this.