The UTM_Content tag is an optional tag for most applications, but many CloudAmp customers find it useful for tracking additional campaign dimensions.
Google Ad Adgroup is a common value to populate the utm_content tag with, so you can see Adgroup data in Salesforce.
Another option is tracking whether Salesforce leads came from mobile, desktop or tablet sources in Google Adwords. If you use utm_content={device}, Google will automatically pass the device if it has device data. Google will fill in {device} with "c"/"m"/"t" and that will be passed to the site and through to Salesforce when a lead is created.