I like that it can be used right away as an online calculator for my personal use at home or in office environment where there is no access available via app (iOS would have been great).
Also very easy way/interface if you want just do some inputs without doing anything else - not necessary reading any pages etc! It's actually quite simple interface but still has all features which are needed by users who don't need more complicated calculators outthere. For me personally this tool helps keeping track how much resources i consume so far per year. Easy tracking when comparing previous years stats vs actual current results makes everything easier than ever before :).