The block of code itself is disconnected from the control statement which allows developers to comment out or remove entirely a control statement without having a large risk of encountering syntax errors because of extra or missing braces.
The code is clearly set apart from the statement by lines that are almost completely whitespace and the fact that the opening braces line up in the same column as the closing braces makes it easier to match them.
Since each bracket is its own line, the Allman style takes more vertical space than other indent styles. This is one of the reasons that a lot of book authors do not prefer to use Allman style in their texts.
Ensuring that control structures don't share lines with the content they define is important. Lines are the "boxes" or categories into which related syntax goes. In the case of a code block, all the syntax related to a given statement goes on the same line. This not only helps visual comprehension, but ensures that line-wise operations (user inputs like a triple-click or middle button click to select a line, delete a line, cut a line, duplicate a line, etc.) remain meaningful and effective.
The K&R style uses braces only when necessary. So for example:
if(condition)
doSomething();
But this can be annoying if you need to put something inside that if statement, because you have to go and add braces to make sure that will be grouped correctly with the statement that was before.
If the opening block statement has multiple lines, it really decreases the code legibility because now you have your code block mixed with the opening statement.
Symmetry in size is important because it provides a visual cue that two things are similar in effect/importance. For example, in written text, lines of a paragraph are the same space apart. The space between paragraphs is larger, but also the same between each. The space between subsections, sections and chapters are progressively larger still. The space between things indicates their relatedness, and helps the reader to implicitly grasp the relationships between elements in the document.
Each line tells the reader something they wouldn't otherwise know, and they can therefore progress from line to line next gathering new information at each without pausing at lines which only contain a brace indicating that a code block is about to begin or just ended.
With this style, one of the advantages is that the opening brace does not take a whole line for itself, wasting a lot of space. The closing brace though is indented on the same level as the control statement it belongs to, making it easier to understand which block of code it's closing.
Indenting the braces makes them feel that they are subordinate to the control statements. Which helps the reader understand which block belongs to which control statement.
The fact that the code lines up up with the braces makes this by far and away the easiest style to read. The code flows much more naturally, there is zero doubt about which block the codes belongs to, your eyes are quickly drawn to what belongs where.
Sometimes a control statement can take a lot of arguments and go over many lines. It frees your mind from doing the work to separate arguments and code block.
When scanning vertically through large sections of code to review the code execution flow, having the braces indented means less noise to filter out. The execution flow statements stand alone on the vertical line and are thus far more quickly picked out by the eye.
The braces confuse the focus of the reader's eye and this causes the reader to look longer at the line right after the opening brace and at the line right before the closing brace.
The GNU coding standard requires that programmers write their comments in English because English is the most widely known language by programmers all over the world. This makes programs written in GNU style easy to understand internationally.
Since GNU programs are designed to be compiled with the GNU C Compiler (gcc) and run on only one system, the GNU system, they have virtually no portability issues.
Since the Kernel style uses 8 characters for indentation, the lines of code are pushed too much to the right. This may make it harder to read for small monitors (for example terminal monitors which are 80 characters wide).
The large indentation suggested by this style makes it easier to define where a block of code starts and where it ends. Especially after looking at a screen for a prolonged amount of time.