Tools such as these are great because you can not only put your own site in there and track its progress SEO-wise, you can also look at other sites and learn from their failures and successes.
Also, each of these tools seems to have a slightly different emphasis. So, like a patient with a mystery ailment getting a second, third or even fourth opinion, you end up with a finer and more nuanced assessment of the state of your website.
Also, they all have explanatory notes attached to each aspect that they're assessing. When combined these descriptions constitute a pretty good roundup of recommended SEO practises. Just reading through these is illuminating - almost like having a brief but comprehensive lesson in basic SEO.