I love that it's easy to use, you just paste your html code into one of those boxes at top left or right side then click "Check" button near bottom center which will give me all possible problems with my codes such as unclosed tags etc..
The results are also very clear too! Not enough features yet but i think they're working hard to make them available soon so don't be afraid if not everything has been included like some other tools might have done (maybe because their price isn;t low). If u want simple way to check any error without wasting time by doing things manually this should do fine :) This can help people who know what kind of problem could happen when writing bad markup/html5 css3 pages especially mobile ones. For example sometimes after adding new feature we forget about closing tag properly therefore causing issues during debugging process later down-the line :D.