If you use this option, when you receive an email reply back from your client, you can save it too under their contact record.
This way you can keep track of all client incoming and outgoing emails in the one place.
Can I set this option by default?
Yes you can have this as a company-wide setting to ensure that all emails sent from the system are saved to the CRM by default, do the following:
Click [My Account]
Click the [Company Settings] tab
Select the [Force "Save Email to CRM] tickbox
The [Save Email to CRM] tickbox will now always be selected automatically when sending a new email from the system.