Notes vs Description Fields
While you can use any Mobly field any way you'd like, the description and notes field were created and placed the way they are with intention.
Description: There is only one description per lead. Any edit to this box will stomp on previous data and overwrite it. In SFDC, the description maps to a field of the Lead Object. This field was intended to be a quick description or relevant context information about the lead. For example: "This is the CMO, but he doesn't have buying power."
Notes: A lead can have multiple notes. They appear in a chronological list on the engagement tab and push to the CRM as a new item in the activity log. In SFDC, this maps to the Tasks for the Lead Object. This field was intended to be more temporal in nature and give detail about interactions with the lead. For example: "I just talked with Jake over the phone and he's interested in buying."