Scenario:
A streaming site has streaming rights to an anime and then a different company picks up the title for DVD/Bluray viewing.
Question:
Does the second company have to re-subtitle the show even though that work has been done already by the streaming company?
Because I assume the subtitles belong to the company who made them - Do companies sell them on maybe?
Some of it may depend on the region/country which the original translation was created and where the second company's region is. In the U.S., translations are derivative works, which are copyrighted separately themselves but cannot be produced without the consent of the original work's copyright holder.
And because the reason for doing the translation and distribution (be it streaming or physical media) is to localize for a region's consumers, there's usually no reason to need a new translation but it ultimately depends on the original copyright holder. Sometimes, simulcast streams have translations that were done under a heavier time constraint, even if the scripts are shared to the translators before the original airing of episodes. There's less supervision in some of these cases and the original copyright holder (e.g. the Japanese Studio) may not have the same control they'll have if they work directly with an editor or a script director for the licensor (e.g. the streaming company or distributor). At the same time, the original license holder may ask that another translation be used instead of the licensor creating their own new translation, though I'm not sure how often that ends up happening.
I don't have any numbers or citations to give you an idea of how many streamed translations end up being re-used by other distributors, but they would either pay a licensing fee to the translation's copyright holder (be it the streaming company or a translation service), or they could just buy it. The original copyright holder, though, would most definitely have some say about this when the license to distribute is hashed out. If the original copyright holder is OK with reusing another translation, then that's probably what usually ends up happening. Otherwise, the studio may demand a new translation with more direct input.
Note that translations, even on physical media getting reused isn't uncommon, especially with distributors for different regions but the same language (U.K., U.S., Australia, etc). Also note that some studios get new translations made for the same property on newer releases (e.g. ADV's Neon Genesis Evangelion's perfect collection vs platinum collection).