Nope, that's not the main purpose: through rSAP (remote SIM access protocol), a in-dash car phone (for ex.) can "grab" the SIM card in your mobile, so no need to have "dual card" arrangement for dedicated car phone. Advantage is that you are using a real car phone, with its own external antenna, steering wheel user i/f, SMS messages store, voice dialing etc. In other words, not just a "parrot" or wireless headset. And yes, some of them can grab contacts on the SIM card as well, or even on the mobile phone.
Sounds all very nice, but in reality it only really works with the following combination: VW or Audi and Nokia phone. My HTC Windows Mobile based PDA in theory should work as well, but in practice is a nightmare. That's why I'm longing for Apple to implement rSAP for quiet a while now; to bad that even in V.3 it still is not there.