According to https://docs.brew.sh/Analytics they use it to measure how often formulas fail to install, to get overall metrics on which OS versions are used, and to correlate those (i.e. to tell on which OS versions specific packages fail to install correctly).
> A maintainer has no need to know who's installing what
Aside from the IP, they don't know who's installing what, and in the new model announced in this post they now don't store IPs or any other user token at all, so it should be purely anonymous aggregate metrics.