How are they functionally different and why are they both necessary?
They are different since one runs before the full boot and therefore has access to more files. As to why are they both necessary? That depends on who you ask. I rely on the Boot Scan and all of the on demand scans and various shields once The boot scan declares a clean system. I do a new boot scan each time avast! issues a new Major release. (To each their own. ) What ever works for you and makes you feel safe. :)