Hex and RGB are 100% equivalent. No difference in terms of how the code is read by the Web browser, both are fully compatible with every modern browser, including Internet Explorer (RGBA, however, is not compatible with old versions of IE).
People who come from a computer science background are likely more comfortable and familiar with using hex in general, as it’s the number format for many things in computer science (memory addresses, op codes, etc).
People who don’t come from a computer science background but do have a background in digital graphics (think Photoshop, Illustrator, and other graphics programs) are likely more comfortable and familiar with RGB, because those programs display everything in RGB.
Hence, CSS is flexible enough to allow people from both backgrounds to define values in the way that they’re comfortable with. It’s really nothing more than that. Neither scheme is “better” than the other, it’s mostly about using what you’re most comfortable with. If that’s RGB, then use RGB, or if it’s hex, then use hex.
The one thing every front-end developer should know how to do is convert between hex and RGB. It’s really simple and if you ever work with a designer, you’re going to need to do so eventually.