Why it might be advantageous for a storage polysaccharide to be a branched structure (such as starch and amylopectin) and a structure polysaccharide to possess a linear structure (such as cellulose)?

A branched structure (i.e. glycogen) allows for more volume of C-H-O, which stores the energy within the bonds. The more energy-containing bonds that can be stored per tissue space, then storage can be maximized without overusing an organism's body mass.
Structural starches (i.e. cellulose) are better linear because they can be stacked and interweaved, creating a stronger more durable structure as compared to a wider, more voluminous branched polysaccharide. Strength and durability can maintain the structures better upon physical or mechanical stress. Cellulose helps plants have more rigid trunks, stalks, and stems because of this physical characteristic.