iWatermark provides watermarking tools not available in Lightroom. For example, a text watermark in Lightroom is one fixed size in pixels so that watermark will vary depending on the resolution of the photos being watermarked. Whereas iWatermark has text watermarks that optionally scale proportionally depending on resolution or portrait/lanscape. Lightroom uses pixels to determine watermark location whereas iWatermark position the watermark proportionally based again on esolution or portrait/lanscape. That means if you watermark a batch of photos of different resolutions and/or landscape or portrait orientations that iWatermark can have a watermark that maintains the same look/identitiy on all these kinds of photos. iWatermark also has options not to scale. These are 2 big differences.