Why does UnderscoreJS have wrapper functions around a lot of native Javascript functions?

Question

I noticed that UnderScoreJS has a lot of wrapper functions around native Javascript functions.

Take for example:

_.isArray, _.isBoolean, _.isNaN?

Is there any reason for this? Or are these simply meant for ensuring code consistency when using an underscoreJS library OR just enhancing these functions in anyway?

For example, the _.isArray function gets down to this:

_.isArray = nativeIsArray || function(obj) {
    return toString.call(obj) === '[object Array]';
  };

Any idea?