what is the idea behind an "ip unnumbered" interface. I know it helps conserving addresses on the network, but when should it be used?

Here is what I did:

int loop 1

ip add 192.168.1.174 255.255.255.248

int vlan 1

ip unnumbered vlan 1

...

I did it in hope that the next time vlan 1 is down I still can reach the router through 192.168.1.174. But after all the reconfigurations the 192.168.1.168/29 net became unavailable. I coldn't ping any PC that I had been able to do before. And another question: how do unnumbered interfaces relate to their "numbered" hosts? Are they bridged or what?

thx in advance.