Why should I build with --with-keep.source

I remember having seen in Hadley Wickham's book that using --with-keep.source while building a package is useful for debugging (I think)
I though that this was related to being able to have access to R function code in the event of a bug but I just installed a package built without --with-keep.source and I see no difference.

Can someone shed some light on this please ?

In order to debug effectively in your package, you’ll also want to ensure that your package is compiled with the --with-keep.source option. This option is the default for new packages in RStudio; if you need to set it manually, it can be found in Tools -> Project Options -> Build Tools.

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So, it is for debugging while writing the package. Even though it is now set by default, if you aren't profiling, or if you're so fortunate as to have no infestation, there's no practical difference.

Thanks would you know what is changing (practically) when the option is on ?