grepmail cache

What is the purpose of the grepmail cache? I'm presuming to allow search
performance to be increased?

Assuming that is so, how do I use it? Today I tried to initialize the
cache by doing what is kind of suggested in the man page ("This is useful
if you want to initialize the cache without printing any output.")
$ grepmail -MRb -e \(\) *
in the directory where I keep my email. It got boring after about three
hours of running without any feedback as to progress. Was I doing the
right thing?

Once initialized, do I somehow have to keep the cache up to date?

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