Although I love many singers, there are three that for me are the benchmarks of singing: Sinatra, Dusty (Springfield), and Streisand - the 3 S's!.
To me, they each hold the key to elements I have personally aspired to (and continue to aspire to) when working on my own vocals. In my humble view (as this stuff is all subjective at the end of the day) any singer, professional or non-professional, can learn all there is to know about the craft from them..
This song, in particular, is one of my all-time favourites of Streisand, and one I practice to: to open up my voice, my spirit, and to work on precision. Not that I'm saying I sound anything like her - if only! But working with songs, even if they don't fit your voice type (baritone, mezzo-soprano etc.), can still be done if you can sing all the notes - just not full voice. I can still mimic what Streisand is doing in order to work on it, and then place it into my own range and material.
So what is going on in this particular song? The control she has in general and, in particular, in long, smooth phrases ("just like me to lose my love..."). Her use of vibrato - knowing when and when not to apply it (the latter being on "glove" and "love" at the top of the song). The way she opens up and soars (1:29, 2:07) and has the control to come right back down again (2:27). The way she can hold a note solid and manipulate it to its conclusion (2:53).
In case you think her skill is aided with studio trickery, take a look at "A Happening in Central Park", a live recording that is just breathtaking: not just for her voice, but the whole package. A star indeed!
Streisand sings fearlessly, which is something we should all aspire to.