Basically a commercial TV operation licenses some content (under conditions like time, region, possible number of viewers etc.) and they try to sell that to as many customers (advertisers) as possible, hopefully with enough markup to more than pay for their licensing fees and operating fees.
And then the TV channel and the customer hope the viewers (in the preferred demographics) show up to actually consume the ads.
Sometimes specific episodes have specific issues like: some licensed content can't be re-licensed, customer (advertiser) has some issue with it, etc. etc.
All of these things can change over time. Most viewers don't keep watching the same stuff forever. If you want something reliably available to you forever, Blu Rays and DVDs are better for that.
By the way, none of the above is specifically related to streaming. This description would have worked in 1950 too. (Except that DVDs and Blu Rays didn't exist yet.)