While the += notation is idiomatic and shorter, these are not the reasons why it is easier to read. The most important part of reading code is mapping syntax to meaning, and so the closer the syntax matches the programmer's thought processes, the more readable it will be (this is also the reason why boilerplate code is bad: it is not part of the thought process, but still necessary to make the code function). In this case, the thought is "increment variable x by 5", not "let x be the value of x plus 5".
There are other cases where a shorter notation is bad for readability, for example when you use a ternary operator where an
if statement would be more appropriate.