In my experience, the number one MAJOR leg-up that Forms Professional has over Essentials is the ability to use database lookups for field population.

So, for example, if you have a Form with a "Vendor Name" field; rather than having submitters manually type the vendor name there (and risk typos), you can have your full list of vendors stored in a database table, and use a lookup in Forms to create that field as a drop-down list, then users can simply select the appropriate vendor from a given list.

You could also have additional vendor information in that database table (i.e. address, phone number, etc.) so when the vendor is selected it auto-populates other vendor fields on that Form with the appropriate information. This gives you the ability to streamline your submission process and create more ease-of-use for Form submitters.