That route tells it to just send packets for any IP directly out the interface, this is usefull because in a LAN using zeroconf LL IPs (as avahi-autoipd), only hosts without another non-LL ip should have an LL ip.
So without a default (last resort) route out the interface, if you only had an LL ip you would not be able to communicate with other hosts that didn't have an LL ip.
That route tells it to just send packets for any IP directly out the interface, this is usefull because in a LAN using zeroconf LL IPs (as avahi-autoipd), only hosts without another non-LL ip should have an LL ip.
So without a default (last resort) route out the interface, if you only had an LL ip you would not be able to communicate with other hosts that didn't have an LL ip.