It's like ParticleSystem getters. Is the functionality useful? Well... maybe... possibly... perhaps?
But! Is knowing that you can set and get the state of objects and the global state useful? I really do think so! You don't need to remember which state can or can't be gotten or set, because it all can; it's simpler.
Edit: For reference, there is SpriteBatch:getBufferSize and ParticleSystem:setBufferSize andParticleSystem:getBufferSize. SpriteBatch:setBufferSize wants to belong.
I suppose it could be useful for stretching the available space in the SpriteBatch (without having to re-add everything), if the exact maximum number of sprites isn't known when it's originally created.
Not all getters should have a matching setter though, although in this case ParticleSystem:setBufferSize already exists which makes the argument for SpriteBatch:setBufferSize a tiny bit more compelling.