Salesforce Optimizer is a great tool, but using it is not intuitive and easy as is the case of Salesforce Org Analytics provided on Elements platform, where you find deepened insight at the top of your hand, few clicks away.

Salesforce Org Analytics (Elements) has a small overlap with Salesforce Optimizer Report. The Salesforce Optimizer Report is a free report that is run from within Setup. It gives some vital signs on the health of the Org and is delivered as a PDF. Type "optimizer" in the Setup search box. Salesforce Optimizer looks at Org limits and has some great metrics to help optimize the Org. The report is over 70 pages long.

Salesforce Org Analytics goes into more detail about how well documented the Org is, such as the Descriptions completed chart. 

Another difference is in the area of custom fields and help on which fields can be deleted. The Custom fields by custom object chart gives you an overview. You can also see the number of records by object. Then you can go into the ref model for that object and look at the %filled for an object by record type (i.e. which have data), and finally the field usage by object.