The enabled interrupts will wake you from sleep (of course, the interrupt may not be able to happen during sleep if it depends on the clock), but if GIE is off the ISR doesn't get executed. Instead, the execution will continue after the "sleep" instruction.
If GIE is on, the first thing being executed after wake up will be the ISR.
This gives you enough flexibility to do what you want, such as wake up with an interrupt you don't otherwise handle.