update README

README.rst

@@ -40,7 +40,7 @@ A mail-sorting tool that is less annoying
 General
 -------
 
-Tabellarius is a mail-sorting IMAP client that depends on IMAP only. Unlike others it uses the `same IMAP connection accross multiple IMAP commands <https://github.com/lefcha/imapfilter>`_ and `simple markup language <http://www.rfcreader.com/#rfc5228>`_ instead of a complex scripting language though it isn't as feature-rich as the well-known Sieve standard.
+Tabellarius is a mail-sorting IMAP client that depends on IMAP only. Unlike others it uses the `same IMAP connection accross multiple IMAP commands <https://github.com/lefcha/imapfilter>`_ and `simple markup language <http://www.rfcreader.com/#rfc5228>`_ instead of a complex scripting language though it isn't that feature-rich as the well-known Sieve standard. It became necessary because of missing features in the ManageSieve protocol and service providers that don't even provide a ManageSieve service or any other *human-friendly* filter techniques.
 
 It is written in Python 3 compatible source code that uses the Python modules ``IMAPClient``, ``PyYAML``, ``backports.ssl`` and optionally ``gnupg`` to parse config files and operate on e-mails via IMAP.