Streaming services provide a single location for the providers specific content for a single monthly subscription fee. So instead of buying the media for each Disney movie you want, which can become very costly, you get it automatically as part of your subscription to D+ once it becomes available. Plus it doesn't get scratched or damaged, so you don't have to worry about replacing it.
Basically we now live in a digital world, physical media is something that will eventually go away completely. It doesn't make the content, ie the art, any less valuable. A good movie or song is still good no matter how you consume it, whether that be through a physical media purchase or watching it on a streaming service. Also watching content on a streaming service is no different than watching it on cable. The only difference is that the majority of streaming services provides content that is specific only to the company offering the service, such as Disney. A few such as Netflix still offer content from other providers, but that is also slowly going away.
As for specific movies not being on D+, a lot of that has to do with existing contracts with other content providers/disturbers on said movie, and its specific to specific regions such as the US. For example Cinderella (2015) in the US is available on Starz and its streaming service, but is actually available on D+ outside the US.
The other part of specific movies not being on D+ has to do with the content still being in the Disney Vault. Disney is slowly releasing that back catalog over time. So old shorts, shows, and movies are being released weekly onto the service. This is done so it provides you something new to watch all the time keeping you subscribed.
There are other aspect of Streaming not discussed here, but I hope that provides you a better understanding of streaming services.