Syncing Activities to SFDC vs Marketo Sales Insight

If you have Marketo Sales Insight installed, which, on the Email tab shows whenever and email is sent AND you have an interesting moment for Fills Out Form, what would be the reason, if any, for ALSO syncing those two activities (Was Sent Email and Fills Out Form) to the SFDC Activity log via the SFDC Salesforce Sync options in Marketo Admin?

Unless I'm missing something - it seems redundant, PLUS, Syncing Activities to SFDC uses up SFDC storage space, which, I presume, is not the case with the Marketo Sales Insight.