The camera and fast updates are the reasons I can see to get the Pixel line of phones.
The camera on the Pixel 3 are excellent, but only marginally better (if at all) than the Pixel 2. Still photo imagery is still great on the Pixel, however some of the features are quite poorly implemented. Top shot sounds good and works well, but the images it takes are such pathetic quality that I find them unusable. They're 1mb photo size and look like a photo sent over SMS or something. The sound on video recording is also absolutely abhorrent. Sounds distant, tinny and lacks volume. Hope both these things get fixed with updates.
The phone gets fast security and OS updates, which is always nice, but even then, I now have reservations. While I loved the Pixel UI back on Android 7.0 and 8.0, but on Android 9.0 with this new half-gesture, half-button UI to me is really quite bad, and I don't like how Google didn't allow the traditional layout, even if they were to bury in developer settings. The new gesture system is much slower to navigate than the previous method. It's clunky, cumbersome and imo looks worse as well because it's lop-sided now. So now I'm not confident the changes Google are making to android are necessarily good.
So back to the question, 'why buy it'? It's got a great camera, and if that is the most important thing to you, then it could be a great phone for you. Otherwise, you get much better bang for your buck with other phones. Better designs, better hardware, better specs, much better battery life (I'm very disappointed with the Pixel 3 XL battery life, it's even worse than the Pixel 2 XL I had) and at cheaper prices.
Honestly, I think unless you're an extreme Google/Pixel fan or you must have the best camera, I think other phones are better overall devices for consumers. I find this is the Pixel 2 XL just with a much, much better screen and a worse UI. I find it hard to justify buying this phone.