Pure distraction. A pipe bomb is what high school boys use to blow up mailboxes. Even if they went off at that hour, placed under a bench, the most they would have done is break a window or give a person a shin frag.
You're kidding right?
An M-80 is what high school boys use to blow up a mailbox.
A pipe bomb is as lethal as the bomber wants to size it. The iron or steel casing, in addition to throwing deadly shrapnel, contains the initial explosion to make it deadlier when it exceeds the pipe strength.
As for "if they went off at that hour", we have no idea when they would have been set to detonate.
And placing them under a bench could easily kill anyone sitting on that bench and or sitting on nearby ones, not just give them "a shin frag".