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Avast is no different to other AV’s , once an infection has taken root you often need other tools to deal with the problem.
Much of the malware, if allowed on to a system , will turn off or interfere with the resident protection.
The tools you refer to allow the experts to see what is hiding on your system, where it is within the registry or system files.
Some of the tools allow you to regain control of your AV or antimalware program .
Others will clean certain problems , used correctly your system can often be saved.
PC shops, repair services use these tools, they just don’t tell you. 
Malware evolves 24/7 , what was caught yesterday, is rewritten and becomes a threat tomorrow. There are 1000’s of new threats created every day of the week. The other big issue is the user … who allowed infections/toolbars/PuP’s past their protection by clicking “Yes” or “Ignore” " Exclude" … oops , my system is infected.