If there's no useful purpose for something, it's unconstructive. It's unconstructive for a teacher to start yelling at the class if the outburst just makes the students even rowdier.
Unconstructive things don't add anything positive or helpful — they're not constructive, useful or beneficial. Unconstructive advice doesn't help you figure out what to do, and unconstructive feedback on your theater performance is either blandly nice or generally critical, without giving you any ideas for how to make it better. We hope you haven't found this explanation of unconstructive to be unconstructive!