Sales managers want their team members to help each other close Opportunities.

 When the organization-wide default for Accounts is private, users can only access the accounts they own or are explicitly shared with them. However, when the organization-wide default for Opportunities is private, users can access the opportunities they own, are explicitly shared with them, or are associated with accounts they can access1. Therefore, when ownership-based sharing rules are created for each team to grant access to opportunities owned by their team members, the sales reps on the same team will also have Read access to the accounts for those opportunities. This is because the sharing rules for opportunities automatically grant access to the parent accounts2. However, the sales reps will not have Edit access to the accounts, unless the sharing rules specify Full Access for the opportunities3. The sales reps will also not have access to all accounts or all opportunities, only those that are owned by their team members or themselves. References: Sharing Records Owned by High-Volume Portal Users | Salesforce Security Guide, Sharing Rules | Salesforce Security Guide, Create Owner-Based Sharing Rules | Salesforce Security Guide