From a user's perspective, when you install an app, you can:

1. Determine if that app is allowed to access the internet.
2. If it needs access to your contacts, you can share which of your contacts, it can see (or none at all)
3. If it needs access to your files, you can determine which files/photos/music it sees (or none at all, but the application still believes it has access to everything)

There are a bunch of other, security features it provides, but from a "normal user" experience, the ability to take control of your data is probably one of the most impactful.

It is possible to do similar things with other CFW, but AFAIK, graphene is the only one to cleanly integrate it as a polished feature of the ROM.

edit: fix formatting

Basically GrapheneOS is for people worried about law enforcement or some state actors trying to access their phone using some commercial tools or 0 day exploits. It's useful for journalist, lawyers, activists and so on.

Average users don't really have to worry about those things. It's unlikely that someone will try to hack you using such tools, you most probably don't have any data wort protecting and it's quicker and easier for you to just unlock your phone than to spend days/weeks/months in jail trying to protect your data.

What average user should care about is removing Google from their phones and blocking trackers. Other ROMs like iode also come without Google and have better tools than GrapheneOS for blocking trackers. They are as secure as any other Android phone.

A big thing is gOS not using JIT compiling. So, app updates are pretty slow but this kills a lot of malware exploits.

https://grapheneos.org/features#exploit-mitigations