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.