That's an excellent question. IIRC, a lot of the CPU testing & benchmarks show it on par with the R5 1400/1500X, so you can probably pair it with just about any kind of GPU you want. You're right, though; if you can afford the 1080TI (especially at current prices), you can probably afford a better CPU as well -- & if nothing else, if you really need the extra PCIe lanes, given the same price you'd probably be better off with the R5 1400 than the R5 2400G.
But, I think these form a nice niche for those that want to have out-of-the-box decent gaming performance without having to also shell out for a decent GPU right now.