Well, you may not need it. But, depending on how you play back, it may be using a lot of CPU power to do the playback. On some systems as bitrates get very high in video it can cause dropped frames and not-so-fluid playback causing annoying stutters. Personally I prefer hardware H.264 video decoding (in the case of playback on the computer, this means using the video card to decode and render the H.264 compressed material) as it tends to offer smooth playback even at extremely high bitrates. Software decoding tends to utilize a lot of CPU power depending on the source material and some people prefer to offload that work to the GPU if possible.
If you're not having a problem with stuttering then you don't need it, if you find that your video seems to have hiccups or stutters during playback then you need a way to use hardware acceleration.
Unless things changed in 10.6.4 that was just released, quicktime only does hardware acceleration on MP4/M4V/MOV that has H.264 video in it, which means other containers don't benefit from hardware decoding.
Here's a tidbit on hardware decoding:
H.264/MPEG-4 AVC - Wikipedia, the free encyclopedia