As with any spell, your Voidwalker's Torment has ranks. You receive these ranks at levels 20, 30, 40, 50, and 60. Just before those levels (*8 or *9), especially as a Warlock becomes more powerful, your Voidwalker will have quite a bit of trouble keeping aggro. This talent makes a huge difference, and many Warlocks consider this talent essential for effective Voidwalker soloing.
Just to note: The improvement of Demonic Frenzy increases each stack by 1/2/3%, making 6/7/8% per application (up to an overall of 60/70/80%), so it's a very noticeable improvement to your Felguard.