In London, before Deliveroo, we had Just Eat and Hungryhouse (and we still do). I would say that the big difference between those two and Deliveroo for consumers is the selection of restaurants: the former offer a huge array of 'normal' takeaways - Indian, Chinese, Thai, pizza, that sort of thing. Deliveroo has fancy stuff - hipster burger places, my local high-end 'modern British' restaurant, the nice sandwich deli down the hill (who offer a roast beef sandwich with "incredibly slutty gravy mayo"). I remember when i realised that i could punch some buttons on my computer and get a fresh crèpe Suzette delivered (from Le Mercury in Islington, i think).
(That said, looking on Deliveroo now, i see a lot of the usual suspects (including such not-quite-so-high-end options as KFC and Papa John's), so perhaps the perception is greater than the reality.)
I think that market differentiation goes hand-in-hand with owning the delivery infrastructure, because those high-end restaurants don't have their own delivery infrastructure. Delivery isn't part of the business model of a high-end sit-down restaurant, or part of the core competency of the people running a hipster burger startup (cooking and managing the Instagram account at the same time is hard enough!). Pure information brokers like Just Eat can't get you food from those places, but logistics operators like Deliveroo can.
I suspect this is why the existing platforms didn't do this - they already have complicated businesses built around the information bit, and didn't want to take on the complexity of also running logistics.
As for why this is worth 900 megabucks, well, that's easy: interest rates are at record lows, and there are tax breaks on venture capital.