The second link in an aggregate pair clearly not going to be adding any kind of performance or processing advantage while unplugged. Beyond the obvious (plugging in both interfaces) the only advantage I could see is the ability to plug in the second NIC and then unplug the first one without the overall logical interface coming down.
The more likely answer is that the upstream network infrastructure either didn't have sufficient port capacity or there was some sort of mismatch in capabilities between said switch (or switches?) and the host. Having both ports plugged in might have broken something and unplugging one fixed the issue, at least temporarily.