Difference between Full System Scan and Boot time Scan

[REDACTED]

A few months ago I did a full system scan and recently a quick scan. Each time a bad file was discovered and was either fixed or chested. However, each time Avast then recommends to do a boot time scan. . .the first one I did. . took forever. . .like 17 hours or something. . .the second one recently was three hours in and was at 11% on the C drive. . .when I pushed escape. . .. . . I have a couple of external hard drives and those take hours to scan as well. . .anyway. .here's my question: If I've done a full system scan. . found errors and corrected them. ..why must I then also do a boot time scan involving hours. . .during which I cannot use my computer because it only boots after the lengthy scan. . which appears to find the same problem and carry out the same solution. What is the difference? Why is a boot time scan recommended after having just completed a full system scan?

[Asyn]

1. What is the difference?
2. Why is a boot time scan recommended after having just completed a full system scan?

1. The difference is that BTS runs before the system is booted.
2. It gets recommended if the full system scan did find an infection.

[REDACTED]

Pardon my ignorance. . .but what is confusing to me is why is a boot time scan necessary if I've already done a full system scan. .have found a problem. . and have fixed or chested the problem. I realize that the boot time scan takes place before boot. . .my question is why is it necessary given the process of the full system scan. .which scans the system, finds and, allegedly, fixes the problem? Why not just skip the full system scan and just schedule a boot time scan? How are they functionally different and why are they both necessary?. . that's my question. . .again, pardon my ignorance if the answer is obvious and I'm just out to lunch. 

[Pondus]

not 100% sure .... but i think the suggestion to run boot scan pops up depending on type of malware that was detected / what places it hide,  to improve removal

what was detected in your case?
what was the location?

[bob3160]

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.