use cases for having both modify-vars and global-vars

I understand modify-vars and global-vars are different, but what are use case for using global instead of modify, on first sight having both seems in contrast with http://lesscss.org/features/#variables-feature-default-variables.

Why should you define a global if you overrule it in the file? Possible use cases are compiling more than one file with the same settings?

For in browser usage the above case now can be solve easily by using data attributes:

<link type="text/css" href="t.less" data-global-vars='{ "color": "red"}' rel="stylesheet/less">
<link type="text/css" href="t2.less" data-modify-vars='{ "color": "green"}' rel="stylesheet/less">

Also task runners can compile a list list of files, but also that cases can possible solved by defining more than one or conditional task, instead of defining defaults (globals).

Review the Less variables feature documentation linked in the issue, focusing on the distinction between modify-vars and global-vars. Determine whether concrete use cases, such as compiling multiple files or browser data attributes, should be documented; done means the rationale and distinction are explained clearly.