I can think of a few reasons, not all of them may make sense or are great though.

Cheaper cells that are less efficient on these tall buildings could generate that extra energy we need to power smaller buildings in the area that may not see much if any sunlight.

Taller buildings get more sun through the day, even if it is only by a minute or two.

The more energy we generate and cam store the less reliant we could be on other methods of energy generation.