Honestly, since I upgraded to a 4K Smart TV, I haven't really double-dipped or re-purchased a lot of films I've previously owned, aside from a select few. But it's still great for the select amount of new movies that I've wanted to add to my collection or watch, as well as for something like Netflix, since pretty much all of Netflix original content is now streamed in 4K UHD. For the extremely easy access to apps and streaming services as well as the picture quality, buying a 4K Smart TV is a no-brainer for me -- putting aside the need/desire to rebuild our film collections with UHD Blu-rays. I can also obviously still watch any and all of the DVDs/Blu-rays I previously owned, so "taking the plunge" is not nearly as disruptive or costly as it once was to make the jump from VHS to DVD.
Aside from picking up some UHD Blu-rays, I've became a pretty avid user of VUDU (which came loaded on my TV along with my PS4). It's a pay-as-you-go service with no monthly fees that allows you to rent or buy movies and TV series. New 4K UHD and regular HD movies often pop up on VUDU as "exclusives" a few weeks prior to DVD/Blu-ray releases. I've built up a small collection of films I own on VUDU, which I can now take with me anywhere I go and watch on any TV or device.
VUDU is also basically Blockbuster on steroids. You can find virtually any film or TV series on there from any time period, as opposed to other streaming services which have a smaller selection that rotates and varies from time to time, or VOD services that primarily only have recent releases available. VUDU allows you to search by genre but also by dozens of sub-genres, which is cool ("Thrillers" separates into categories like Detective/Police Thrillers, Murder & Mystery, Psychological Thrillers, Classics, Techno-thrillers, etc). You can also filter by release date, most watched, top picks, and even by Rotten Tomatoes scores.
Since it's a pay-as-you-buy/rent service, it makes for a nice supplement to other streaming services like Netflix/Hulu and to making physical Blu-ray purchases for your more "important" movies to own. It also probably has the best and biggest collection of 4K movies in one place with easy access.