Without PPPoE Passthrough, your router would dial 1 PPPoE account itself, but it won't allow any PC or network device connected to it to dial additional PPPoE accounts.
The reason why you want to dial more than 1 would be to do traffic splitting. The usual traffic splitting method involves 1 account with international cap & another with local cap. The traffic splitting script/application would then route all the local traffic through the cheap local only account and the international traffic through the international account.
Other traffic splitting stuff involves like unshaped accounts and routes to specific servers, like when you buy a quite expensive unshaped account simply to be used for playing WoW on overseas servers.
Alternatively, multiple accounts can also be used to allow different people to share the same ADSL line while having their own accounts. Using this method, you would actually want to put the router in bridging mode, otherwise the people could just stop dialing their own accounts and use the one dialed by the router. In bridging mode, it will always allow PPPoE passthrough - even the cheapest routers would.
