I do nothing with my expensive phone (Pixel 1 XL atm, soon to be iPhone X). Nothing to justify the price, at least. I buy them because I enjoy the quality feeling, which is partially real and partially self enforced.
With that said though, I've been with Android since the G1 (literally all of the Nexus lines), and few cheap phones compare to the high end phones for me.
My best phone of all time? The Nexus 4.. or was it 5... I honestly can't remember now. It was one of these $200 phone. It felt cheap as hell, but it was a very nice compromise on features vs price. Yet, that's going back to what I was saying above. Despite me loving that phone, and it being an amazing phone for $200, it was still a compromise of quality. If you show me a phone that has the OS and build feel of my Pixel 1 XL, I'd happily buy it. However, I've not ever seen this. Same goes for my Macbook Pro. It's far from perfect, and definitely feels overpriced, but the OS and Hardware combo has no competition from my perspective. Unfortunately.
So back to your original question, I definitely don't do anything with the phone to inherently justify the price.