Well writing certain elements in pure Galaxy code can simplify the implementation for the really complicated portions of your map. Of course, in SC2, with the more enhanced abilities of the data editor, complicated triggers are not as necessary. In WC3, some things were almost impossible to do without actually delving into JASS2, but even in SC2, for coders it's often easier (and definitely faster) to just write systems in code instead of mucking through a GUI system.
So far, I don't feel the need to recommend learning Galaxy to make triggers in SC2. If you're comfortable with the GUI and uncomfortable with coding, stick to the GUI. It's more powerful than in WC3, with the ability to define custom functions and any amount of local and global variables, and your own structs and whatnot...
Well, the most important thing to me is the much higher writing speed.
Even if I know exactly where everything is in the GUI, I'll still need relatively long to select everything, to load all this lists with variables and functions and so on.
When I can just write everything down without having it to seach somewhere it's much easier and so much faster - given the fact that I know what I have to write :P
Let's take some math as an example. I want to add some numbers and variables. In GUI I'd have to select each value and add them via the arithmic function.
In Galaxy I just write
It might only save me a second or so, but given the fact how much we have to work with triggers that amounts to minutes and hours.
Also, I am used to programming languages like C+ + or Java, so I'm very familiar with the look and way of working of Galaxy right off the bat.
Next is the better overview. Actions in GUI sometimes are so horribly long that you have to scroll back and forth, left and right to see the entire function.
The same function. But so much shorter. Of course the parameters look more cryptic in Galaxy, but you get used to it.
Especially long and complex triggers become easier to read. You can also format your text just like you want to, which is impossible in GUI.
In general you have more control over your triggers. Even though GUI can do most of what pure Galaxy can, there's still quite a couple of things that are easily done in Galaxy, but more or less impossible in GUI.
And you can code more efficiently in Galaxy. Not by a huge margin (since GUI does a good job there), but still more efficiently (performance-wise).