I am not a firewall expert, but what I have been told that even the experts have different opinions on the practical benefits of an outbound firewall in a home user environment.
What I understand is that the balance between flexibility and security is a moving pendalum. For flexibility reasons Windows has Dynamic Load Libraries which can be called (and even injected) into processes at run time. This flexibility mechanism has numeroeus security downsides. Malware could inject a DLL into a legitemate program and fool the firewall (because the firewall had an allow rule for that legitemate program).
A DLL is just an example, ActiveX, Macro's, Javabeans, Javascript executed through Eval are other examples of flexibilty mechanisms which have considerable security downsides. Even tiny bits of flexibility (e.g. metadata containing a few bytes of code in images) have been misused my malware writers in the past. Therefore firewalls have turned into FW+HIPS to prevent misuse of legitemat flexibility mechanisms (setting hooks etc).
Because HIPS are as effective as the response (knowledge) of the user who reads allows or blocks the HIPS-prompt, the 'legitemate versus flexibility grey line boundery' protection moved to behavioral monitoring, machine learning, reputation evaluation with cloud whitelists. Comodo Firewall for example still has a HIPS, but CruelSister advises to disable it and trust the Comodo reputation service and plus sandbox containment to prevent malware touching the real system.
The reason why
@Trident posts that he has no evidence that the protection of Avast firewall (which is in theory better), is also more effective in practise is, because there are so many ways malware can go outbound undetected in the piggy bag of a legitemate process.
EDIT: I have ran Avast FW with Defender for a while until an acquintance of mine noticed I was using it and explained that I was better of blocking LoLBins in Windows FW (like
@Andy Ful firewall hardening does), than using Avast FW (with a blacklist). The recap above is what she explained me.