Contrary to initial intuition, it's actually to increase the brightness.

LEDs can be driven at a constant current, or they can be driven with a pulsed current.

With constant current you have to limit the current to a relatively low value - for instance many common small LEDs are limited to a constant current of say 20mA.  That gives good brightness for indication purposes, but it's not that great.

LEDs, when driven with pulsed current, can be driven with a considerably higher current - maybe 5 to 10 times as much, or even more.  That could be say 100mA for what would normally be a 20mA LED.  However, there are restrictions on what the pulses can be - typically with limits on the frequency and duty cycle - maybe as little as 1% duty.

The end result is that the higher current increases the perceived brightness of the LEDs, since more photons are being emitted when they are on, but at the cost of some flicker, which is only really noticed when the LEDs are in motion.

So you get more perceived brightness from smaller and cheaper LEDs without using more current (often less current) on average than if they were on constant.

Why is PWM used to control LEDs even when they're only used On or Off?
I'm getting some errors when I try to compile my design in Aldec's Active-HDL. 

# Warning: ELAB1_0026: BITADJ128.bde(BITADJ128.vhd) : (79, 0): There is no default binding for component "buf". (No entity named "buf" was found).
# Warning: ELAB1_0026: BITADJ128.bde(BITADJ128.vhd) : (157, 0): There is no default binding for component "INV". (No entity named "INV" was found).
# Warning: ELAB1_0026: BITADJ128.bde(BITADJ128.vhd) : (277, 0): There is no default binding for component "GND". (No entity named "GND" was found).


I have added these items to the library multiple times, and in different ways, but it still gets hosed up. I'm wondering if anyone else has had a similar issue? I inherited a large design I'm converting from EDIF to VHDL and switching from Virtex-4 to Virtex-5, and there seems to be a symbol resolution problem. 
