What is the difference between ‘mapleader’ and ‘g:mapleader’ in Vim?

If the aforementioned statements are both located outside of function definitions, they have the identical effect of setting a global variable. However, if the first statement, without the g: prefix, is used in a function body, it defines a variable local to that function. See :help internal-variables and especially :helpg In a function:.

Hence, outside function definitions one can access the global map-leader variable simply as mapleader.

let mapleader=","

as stated in the doc.

Why would you want to use g:mapleader? g: is mostly used in plugins to let global variables in order to expose settings to other plugins or for you to play with in your .vimrc.