You want to know the difference between the Owner and the Owner (Sales) on a Lead.
SAP Sales Cloud
Owner is the Employee Responsible for the document. In a WFR, this is the Marketing Employee Responsible.
Owner (Sales) is the Sales Employee party. In a WFR, this is the Sales Employee Responsible.
The Owner Sales is responsible for the Sales. So it could be that a person ABC (ABC represents the Leads owner; = Employee responsible of the Lead document) is the person who creates the Lead contacts, say at a trade fair or similar. ABC would then be in charge of creating that contact/account business-relationship. It is possible, that in your scenario this is the same person as the Owner Sales, in which case you wouldn't necessarily need to have the two fields, it depends on your business's strategy.
You can then also have a second person DEF (DEF represents the Lead's Owner (Sales); = the Sales Employee party) in charge of the Sales.
For example: ABC meets and creates the contacts and gets them interested to do business with you, then DEF takes over to do the Sales.
In the Business Configuration > Fine Tuning > Leads > Maintain Involved Parties you can edit the determination of both the owner and the owner sales.
Owner, Owner Sales, Leads, Marketing Employee Responsible, Sales Employee Responsible, workflow rule , KBA , LOD-CRM-LM , Lead Management , Problem