Board Thread:General Discussion/@comment-25266931-20170224140119/@comment-26397825-20170228064645

It's enemies that force you to grow as a person, that push you and force you to be stronger than you were. Hence Weiss summons the Knight from the White Trailer; she had to push herself to defeat it or her father was going to keep her in Atlas and she would have never escaped his control.

It hasn't been stated that they can only summon Grimm, but so far we have only seen Winter summon Nevermore's and a Beowolf.