Reiner and Bertolt are desperate to capture "the coordinate", the founding titan. But according to http://attackontitan.wikia.com/wiki/Beast_Titan , the Beast Titan
"is one of the strongest of all Titans, possessing the ability to guide and direct mindless Titans and creating new Titans from common Eldians. These abilities are said to be nearly akin to those of the Founding Titan"
Then why do people from Marley need so desperately the founding titan? If it's for its abilities to command other Titans, couldnt they use the Beast titan instead for that purpose?