> There is really need to do this is many cases though. If all you are looking for is connectivity redundancy you can just take the default routes from two ISPs and configure floating static routes on your router and not worry about BGP at all.

That only gives you outgoing redundancy. You can talk to the world with one of the links down, but the world can't necessarily talk to your IPs with the link down.

BGP lets you advertise IPs to the world and what links they're reachable on. You can announce the same IPs on multiple links to multiple ISPs. This is what makes the internet "route around damage".