What doesn't matter? I don't understand.
A trademark doesn't protect a product design from being copied.
It certainly could if enforced (whether enforcement would occur where knock-offs would likely be manufactured is another issue...) If you look at USPTO application #77303256 you'll see that it is for the
design of the iPhone: "The mark consists of the configuration of a handheld mobile digital electronic device"
Regarding enforcement, trademark protection accrues with actual use, however a trademark registrant can sue in federal court (and possibly recover damages for willful infringement)
This trademark registration might be useful to Apple both to deter others from producing similarly configured devices as well as to prevent unauthorized accessory manufacturers using realistic representations of the iPhone in their marketing and sales efforts (as matticus008's indicates in the post above)
Why didn't Apple register earlier?
It may be a combination of Apple wanting to submit a strong application, which must include (within 6 months of filing) "specimens" of use (and apparently 170 were submitted) and the fact that time was not of the essence.
Moreover, that collection of iPhone "specimens" is strong evidence of secondary meaning, i.e. that the public associates the trademark (in this case the design of the device) as specifically being an Apple iPhone. This association of trademark to source is essential for design trademark protection.