For Sales Insight (MSI) each sales rep typically needs a license. The MSI license provide the Marketo tab in Salesforce which to prioritizes leads and give insight into "hot leads", helps you to see activity, interesting moments, see recent web activity, etc... as well as controls the plug-in. It enables Marketing to create pre-defined messages in Marketo (templates) that the sales team can send - like for a mini-campaign or invite to an event, tradeshow, webinar, etc.... And sales reps can always to compose messages on their own too. MSI will also record these interactions as well and sync that to the Marketo activity for the record.
Typically in Salesforce the reps can send messages to their contacts and leads without MSI.