Bsd-mailx is the traditional simple command-line-mode mail user agent. Even if you don't use it, it may be required by other programs.
The /usr/bin/mail command provided by this package is a very simple one, and does not include many features that can be found in more advanced mail user agents.
For example bsd-mailx DOES NOT support: - MIME (i.e. no attachments, no UTF-8 or other charsets support); - SMTP protocol (/usr/sbin/sendmail interface only is used); - POP3/IMAP (bsd-mailx reads local mailboxes only); - maildir format (traditional mbox only format is supported).
If the above features are needed, please consider installing another package providing similar /usr/bin/mail interface: - s-nail package, - or mailutils package.