@terziyski Thanks for the info. The diagram shows 2 PCs accessing the router, and the router having 2 lans to a NAS. So I can see the 2 LANs to the NAS as getting to 2GB/s but why is this aggregation ? As simply using 2 LAN cables from the router to the NAS will do exactly the same.

The PCs on the picture only have a single LAN to the router, and the router only supports 1 GB/s LAN ports, so even if the PC LAN port supports 2.5GB/s, the PC can not access more than the 1GB/s LAN output on the router.

If there were 2 LAN ports on the PC (which I have), and 2 on the router, and I could connect my PCs 2 LAN ports to the router, then I could see how 2 GB/s could be available to the PC - if you were streaming 2 different files from the router.

OR if there was a 2GB/s LAN port on the router, and I could connect one of my 2.5GB/s LAN ports to it then I could also see the 2GB/s to the NAS as useful, as then my PC could handle >1GB/s on a single LAN, so eve naccessing the same stream/file would be beneficial.

So it seems aggregation means simply having the concept of a single 2GB/s ethernet container on the router, but I dont see any point to that, as you have to use 2 cables from the router to the NAS anyway both at 1 GB/s, and everything leaving the router is 1GB/s max, as the aggregation is not to any PCs.   

@terziyski Hi, Thanks for the ansers, you said above "You'll benefit from Link Aggregation only when accessing the device from your LAN.", and now you say "When the PCs are connected through 5GHz wireless network to the router it would be beneficial for sure.". So does this mean I get a benefit when I have the router connected to a NAS with aggregation (2x1Gbps), and then both PCs using wireless 5GHz at the same time ? As you mentioned each wifi can have 2.1 Gbps so from PC to router this is the speed, and therefore both PCs would be trying to access 4.2 Gbps together, and the router could then aggregate to teh NAS and get 2Gbps from the NAS ?

So without a NAS there is no benefit to using aggregation in my entire local networking system, be it wifi, cabled LAN, single PC (with 1 LAN or 2 LANs), 2 PCs etc 

@terziyski Many thanks for all the answers, greatly appreciated.

I was hoping the aggregation could be between my PC and the router, so on my Windows PC it has 2 NICs, and I wanted to connect them both to the router. Then turn on aggregation on both the router and the PC, giving me 2 x 1Gbps connections between router and PC. Of course there would only be a benefit if my PC is requesting a mixture of internet and file across the network, as the router only has a 1 Gpbs connection to the internet - so not point in trying to get more than 1 X 1Gbps through the LAN port for internet only. 

I think I will upgrade to a faster than 1Gbps LAN when they become more available.