What is the purpose for having usernameLowerCase rather than set lowercase: true in a schema?

[leoyli]

in the user manual I found this option:

usernameLowerCase: convert username field value to lower case when saving an querying. Defaults to 'false'.

As the title, what is the purpose for having usernameLowerCase rather than set lowercase: true in a schema? I think this option is redundant since mongoose do it, isn't it?

So, I just curious if there is any special case we should use usernameLowerCase option instead, otherwise I will happy to see it removing from available options.