onMouseMove only fires when mouse is pressed

[thomasjonas]

In the source I found the following code for handling the mousemove event:

onMouseMove: function (event) {
    if (window._blockMouseEvents || !this._mouseDown) return;
    this.processEvent(event);
    this.props.onMouseMove && this.props.onMouseMove(event);
},

Why is it necessary to check if the mouse is pressed (!this._mouseDown)? I would like to listen to mousemove events when the mouse is not down which is not possible now. Can somebody try to explain the reasoning behind this?