Charge is important to a Zarya, she does insane damage at high charge, and she gets her ult way faster. But do not waste all your bubbles for change, then you wont have it to save a teamate when they are in trouble.
She needs good game evaluation. Like for instance if the teams are in a poke fight, there might be a teamfight incoming soon. And you want to save your healers in that teamfight. Spesially now that you have an unique way to protect Mercy while she resses.