There are many reasons for having two ports including having two separate networks. As to why Apple does what they do you'd have to ask them. However, aggregating links doesn't require two Ethernet ports but it does require having two incoming services, at least for what you were asking about in your original post.
Aggregating links was something some people did when we were back in the days of dialups and DSL services that only provided 128 Kbit/s services, but that's no longer the case now that DSL and cable provide much faster service. I'm not even sure you can get a modem that operates much faster than 20 Mbits/sec. I have the latest Motorola 5120 modem which is rated at 20 MBits/s. Much faster than any DSL or cable service currently being offered. Even Verizon's FIOS service isn't yet that fast. Faster services are probably on the horizon but will cost users a lot more than what one pays today. Most ISPs charge extra for their highest speed connections, and not too many typical home users pay the extra bucks for it. A T1 line is only 1.5 Mbits/s which is slower than your DSL or cable service. A T3 line is around 44 Mbits/s. But having either would be a bit expensive, especially the latter, and require special hardware and cost to bring one to your home, so their main use is for businesses. The main advantages are they are dedicated (no one else is sharing the service) and the speed is both upload and download. Home DSL and cable provide much slower upload speeds than downloads - typically 256 or 512 Kbits/s.
