Why stream?

 I am not sure what your issue is.  You enjoy a streaming service such as Spotify, for sound quality as well as for catalog and ease of use.  What is the issue?

Believe I was misled by a sales pitch implying higher-than-USB fidelity. Thanks to you I now see that streaming hi-fi has only been playing catch-up, and everybody else knew this. Gracias for the orientation, which is exactly what I had come looking for.

Hi-Fi streaming is here....if you have access to Qobuz streaming, sign up for a trial. They offer the best quality (upto 24bit/196kHz) streaming at this time. They may not have the vast catalog as Spotify but they are ramping up their library since their US launch. 
To answer why stream.....if you don’t care about exploring new music and content with enjoying your ripped collection and then streaming doesn’t make much sense for you.  

Not trying to be pedantic, but the word streaming can mean a few different things.  One can stream their own physical content that has been transferred to a hard drive (CDs, downloads, digitalized lps or cassettes, etc).  One can also pay a streaming service.  The service owns the content and a fee is usually paid monthly to access their catalog.  You seem to be only using the second definition.
  I gather that what you are saying is that a salesperson convinced you that a paid streaming service will sound better than the best reproduction of physical media.  As laltk is mentioning, there are are different qualities of paid streaming.  Most streaming services use a a compression scheme, to save on bandwidth.  Some services are of offering higher quality streams that approach or in the opinion of some individuals exceed the best reproduction of physical media.
.  The first services around 20 years ago used very severe compression.  When mp3 came along it revolutionized streaming (Apple and Spotify both use different compression schemes that are similar in quality to mp3, so the term mp3 is usually used to refer to all of them).  This is because most people cannot tell the difference between mp3 sound and a CD, even though mp3 discards over 90% of the Audio data contained on the CD.  This then can be the source of many a marketing pitch, as mp3 does sound a lot better to most people than a source such as YouTube, which may perhaps shock a listener of our generation (assuming we are contemporaries, I am 61) in that it has been the most common way for people to access music over the past decade.   Audiogon is an audiophile site, so it caters to those of us that can hear a difference between compressed and uncompressed sound.