They got assurance that Austrasia would be governed as a separate kingdom from Neustria. Which implied continuance of their customs and their private holdings, and not having to share influence at their court with Neustrian nobles. This was especially important to Pepin of Landen and his son Grimoald, who were in a commanding position in Austrasia but whose power would be diluted if they also had to contend with Neustrian rivals. Dagobert tried to sideline Pepin by keeping him hostage so they wouldn't benefit directly. It was probably an attempt, then, to appease other Austrasian nobles so they wouldn't support Pepin.
Dagobert's maneouvre could not prevent the Pippinids building up sufficient power to attempt to install Grimoald's son on the Austrasian throne a generation later. The attempt failed but even within one generation the same family returned to power as mayors of the palace, only this time over all of Francia. And they did eventually seize the throne.
