Assimilation of Picard allowed the Collective access to all of his experience and knowledge including that of Federation and Starfleet tactics and technology. It is likely that this was a contributing factor to the overwhelming Borg victory over the Federation in the Battle of Wolf 359. On the other hand, after being rescued, Locutus' data link to the Collective allowed the Enterprise crew to force the invading Borg into their regenerative mode and activate the self-destruct mechanism of their vessel. Also, his knowledge of the Borg helped Picard destroy the Borg cube in the Battle of Sector 001, thus bringing victory to the Federation.
"I am Locutus of Borg. You will be assimilated. Resistance is futile. You will disarm your weapons and escort us to Sector 001. If you attempt to intervene, we will destroy you."
"I am Locutus of Borg. Resistance is futile. Your life as it has been, is over. From this time forward, you will service... us."
Although the concept of a Borg leader was, at the time, almost a contradiction in terms, Locutus was intended to provide a voice for the Borg, as part of a Borg plan to assimilate the Federation.
The capture of Locutus by the Enterprise-D personnel gave them access to the Borg collective consciousness, and they were able to defeat the Borg by exploiting this access.
Locutus was to have been a voluntary counterpart of the Borg queen, with a mind of his own, not just another Borg drone.