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At the same focal length, apart from speed and maybe weight, what other advantages does a prime lens like the 50mm have over a zoom lens like the 24 - 70mm?
To put it better, If I set my 24-70mm lens at 50mm and I don’t care about length and speed, what advantage would just using the 50mm offer me over the 24-70mm
A prime likely still has several advantages over a zoom at a given focal length. (Well, depending upon your needs and habits.)
A less complex optical design. All else being equal, a less complex design is likely to have fewer compromises, which means the prime is more likely to have less distortion of any sort -- pincushion, barrel, coma, and chromatic abberations. And on the flip side, a sharper lens.
Maximum aperture: an f1.4 prime is two stops faster than a "pro" f2.8 lens. 3 stops faster than a consumer-aimed f4. That lets you use a shallower depth of field and see in dim light much more easily.
Focus speed: a less complex design almost definitely means less weight to move to bring the lens in focus and a larger aperture lets in more light to make focus more sure-footed, so the focus speed can be faster.
I wasn't sure what you meant by "speed." Focus speed or aperture speed? I covered those. Here's one more: composition speed. With a prime I don't need to twist the zoom ring to get to 50mm -- I'm always there!
When comparing to a consumer lens, the prime also typically has a focus scale and a much smoother focus ring (with a longer throw, too).
Prime lenses use simpler optics with fewer glass elements. They don't need the ability to change their focal length, so fewer compromises have to be made with the design. This allows for cheaper and higher quality image reproduction. Less chromatic aberration, more sharpness, better color reproduction, etc are all made much easier because of the fewer design constraints.
That said, a very high end telephoto will still beat a low end prime, but my high end ($2400) 24-70 f/2.8 L II lens from Canon is still beat slightly by the image quality of the midrange ($350) 50mm 1.4. It is close, but the 1.4 has the advantage for most measures.
Prime lenses almost always give noticeably better image quality. Zoom lenses generally are at their best in the middle of their ranges - only the very best zooms give consistent quality throughout the zoom range.
Prime lenses also generally have larger maximum apertures, even lower end primes. The largest aperture you are likely to find on a zoom lens is f2.8, and then only on expensive examples, whereas even the cheapest primes will go to f1.8.
Everyone has covered image clarity, aperture, and F-Stop, but there's three more advantages as well:
A basic prime lens (Canon 50mm 1.8) costs $99 and is a fantastic value. You can buy a cheap prime to experiment with to see if you like prime lenses, before you commit to getting higher-quality prime lenses. There are no zoom lenses cheap enough to throw away, you'll have to rent.
Primes can fill out your kit until you commit to bigger budget. I love the Canon 24-70 f/2.8 L II lens, but you shouldn't buy that expensive a lens until you find you need it often. Instead, using a 35mm prime lens and a 50mm prime lens can cover your indoor, low-light shooting.
Physically, prime lenses are short and light. You can add several to your kit without having to buy a bigger bag or get into better shape.
Prime lenses are more efficient with light than zoom lenses, so in the same amount of light, you can use a wider aperture (lower F-stop), and get a blurrier background. Or a faster shutter speed, and get less motion blur. Or a lower ASA, and get fewer image noise artifacts.
I love prime lenses! When you get a few primes, you should practice switching lenses smoothly. I recommend this article: http://strobist.blogspot.com/2006/06/pro-tip-how-to-quick-change-slr-lens.html
