"Just" buying distribution rights can be no small thing to an independent production. In this context, distribution = publication ("to make public"). Without distribution there's no audience. And that, of course, is the point of making a show (well other than Bialystock and Bloom's scheme to produce a guaranteed Broadway flop). Without an audience there's no money, no fame, and thin prospects for making the next show. The audience may pay-as-they-go (theatrical release, downloads, DVD/Bluray), or the audience may be pre-paid from subscriber revenue (cable/streaming). And, of course in most cases film distribution is a combination of those.
CODA's filmmakers had some very nice things to say about Apple. Some distributors are going to simply buy something and exhibit it unchanged. Others are going to demand "improvements." Some will just throw it against the wall to see if it sticks, others will spent time and money on promotion and marketing. Generally speaking, the more a distributor invests in promoting the film's success, the better chance the filmmakers will have of making another film with a higher budget, even better marketing, a bigger audience, and of course a larger paycheck.
No, Apple did not produce this film, which is why there was nobody from Apple up on the Oscar podium. The finished film's qualities were clear to those who saw it screened by Sundance, hence the bidding war and the high price paid by Apple (and the principal purpose of Sundance is to connect independent filmmakers with distribution opportunities). Considering Amazon was the #2 bidder you can't say that it's just because Apple has more money than anyone else. Apple and Amazon have both invested far more in their Foundation and Lord of the Rings series (respectively), and undoubtedly have far more to say about the production of those projects. This is "merely" a case of a "small" film being recognized for its merits and given a showcase.
Overall, this showcase has worked out very well for everyone. There's glory for all involved, which will pay off in different ways depending upon who they are. For the filmmakers it translates to opportunity to make future projects with better funding. For Apple, which sells Apple TV+ to a worldwide audience (and not only to people who own Apple hardware)... The U.S. subscription price of $5/month translates to $60/year/subscriber. If the success of CODA results in a worldwide increase of 500,000 subscribers, CODA is bought and paid for - a successful advertising campaign. Considering Apple TV+ has an estimated 22 million paid subscribers... that's just a bit more than $1 spent per subscriber from Apple's existing revenue stream. In other words, CODA has already been bought and paid for, so any boost to the reputation and subscribership of TV+ is gravy.