Ask yourself this question: How does it work out where to route the WAN1/2 outbound traffic in the first place?
Src-nat happens at the very end of the chain, if you look at the packet routing diagram, so obviously a routing decision needs to be made earlier somehow.
WAN stands for Wide Area Network and this is the network that connects your router to your service provider. The WAN port usually connects to a DSL modem, cable modem or fibre media converter. LAN stands for Local Area Network and this is the network that connects the devices within your home/business to the router.