The USB specification requires that a host port be able to source 100mA at a nominal 5V on the VBUS pin. That much power is barely enough to allow some devices to enumerate on the bus. (Early versions of the popular Cypress EZ-USB FX2 required a waiver because they drew slightly more than 100mA during enumeration.)
Of course, there is also an elaborate power management scheme that permits the host to shed loads by turning off ports individually. (I've never personally seen power management implemented on individual ports: on systems I've examined carefully either all host ports are powered, or none are. Your mileage will certainly vary.)
In particular, whether your ports are powered when the laptop is sleeping is more than a little OS, platform, and configuration specific.
A device is permitted to draw up to 100mA without asking permission if VBUS is present. For a device to consume more than 100mA, it is supposed to have permission, and to be able to gracefully handle being denied.
Similar rules apply to hubs, with complications for bus-powered hubs which are permitted to restrict downstream devices to only 100mA, while never consuming more than 500mA from the upstream port.
One reason for an external device to include a second USB cable for power is that effectively allows it to double its power budget.
Edit: I weakened the implication that PCs don't manage power per port. Just because I haven't actually seen it happen has little bearing on whether it is found in the wild. The white-box PC with an MSI MB that I was last actively developing a USB device driver on had fairly limited power management capabilities. The brand new Dell on my desk seems to turn off individual PCIe cards under some conditions, so the world of power management in PCs has been advancing (or at least getting more complicated) steadily while I wasn't looking.