Why have Kira, Thomas and Jo been taken to the Edifice in Gathering Blue?

Kira, Thomas and Jo are possessed of an almost magical
talent that gives them the ability to, respectively, weave incredibly intricate scenes
with beautiful colours, carve amazingly detailed depictions into wood and lastly, sing
incredible songs. They have the ability, through their talent, to predict the future and
what will happen to the world. Kira's job, after all, after finishing repair jobs to the
Singer's jacket, is to weave the future onto the blank space on the jacket's shoulders,
just as Thomas will carve the future onto the staff and Jo will sing it. Note what we
are told in the final chapter:

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The three of them--the new little Singer who
would one day take the chained Singer's place; Thomas the Carver, who with his
meticulous tools wrote the history of the world; and she herself, the rone who coloured
that history--they were the artists who could create the
future.

For the Guardians,
then, who are obsessed about maintaining control and preserving their rules in the
society that this story is set in, to control the future means to maintain their power.
They have found a way "to steal and harness other people's powers for their own needs."
Their manipulation and abuse of power is forcing the children to "describe the future
they wanted, not the one that could be."  So, because of their ability to "create" the
future, Jo, Thomas and Kira are incredibly and vitally important, both for the
preservation and the future of humanity, but also for the cynical Guardians, who intend
to use them to maintain their power.