It depends on the Mac in question.
If it accepts a PlainTalk microphone, you need to either use an Apple PlainTalk Mic or an external Mic preamp. PlainTalk Macs don't actually have a Mic input; they accept line-level only.
A PlainTalk Mic has a preamp built in to bring the Mic level to line level.
The Analog-to-Digital converters on external USB devices are superior to the built-in AD chips used on desktop Macs; for the most part they offer 24-bit sampling and better quality converters.
Exactly how you could have a "better" converter for a digital device is difficult to understand if you belive "digital is digital", but that's not the case. Even assuming two similar chips are identical in sample rate and sample frequency (ie 16bit/44.1 Khz), the linearity, error correction, and especially high-cut filters vary dramatically. A filter of the type necessary for digital conversion is very difficult to make; if you assume that the filter should be inaudible, a better word might be "impossible", at least for a 44.1 Khz (ie "CD quality") sample frequency. * Thus the move to 96 and 192Khz sample frequencies.
We would all love to have "audiophile" quality AD/DA converters on our Macs; unfortunately the best chips cost mucho dineros. I have a soundcard that costs $800, and there are converters out there that are better and cost much more.
Another major advantage of an external converter (like an iMic) over the one in any computer, including my soundcard, is it is much more resistant to Radio Frequency Interference (RFI). The electrical enviorment inside a computer is very dirty and almost certainly will add noise and non-linearity to an audio signal. Shielding can reduce this hash outside the computer's physical box, so placing the audio circuitry outside the box is very helpful. If it matters, Macs have much better shielding than nearly any available PC.
Now, if all you want to do is talk to your Mom with a mic/headphone set over the internet, or to create mp3's, these things don't matter much. But, you asked.
The iMic is also useful for those that have a Mac without an audio line-in or built-in microphone jack. Apple dropped the audio line-in on a few of their Macs, and then started putting it back on the newer Macs, due to user complaints.