Doesn't beg the question. There's a whole evolving history there. Reddit didn't need to 'keep users on the platform' because they weren't leaving it. Most images on imgur shared on reddit (and a billion forums elsewhere) were direct links to the images, they weren't going to any sort of imgur landing page or whatever. Imgur just slowly developed that later and surprisingly developed its own also-ran community onsite.

Similarly over on twitter without any image hosting capabilities let third party sites like twitpic host images for years before evetually developing their own service. Twitpic eventually caved under the load of hosting so much stuff without income and it was good that Twitter was successful enough to take it all on directly.