Meaning of double square brackets in bash [duplicate]

Answer 1

[[ ]] are equal to test keyword.

You can also use [ ] but [[ ]] isn't compatible with all shell types

To me, if I understand the real sense of question, question itself isn't well-formulated.
Those brackets have to be here not so much for order determination, but because the bash synopsis require them

Edit

Question is changed, so my answer too.

[[ ]] is used against [ ] because you don't have to worry about quoting the left hand side of the test that will be read as a variable.
Moreover, < and > doesn't need to be escaped

Answer 2

[[ is a much more versatile version of [ in bash.

For example, Using the [[ ... ]] test construct, rather than [ ... ] can prevent many logic errors in scripts. For example, the &&, ||, <, and > operators work within a [[ ]] test, despite giving an error within a [ ] construct.

also, their implementations are different

-bash-3.2$ type [
[ is a shell builtin
-bash-3.2$ type [[
[[ is a shell keyword

Refer here for a good explanation