Brochas daf Chof Vov amud beis

R. Yosi, the son of R. Chanina, said that the Avos established the (three daily) tefillohs (of Shemone Esre)

The Gemara discusses whether the Avos established the three tefillohs, or whether they were instead established post Churban by the Anshei Kneses haGedola as a means of symbolically acknowledging the daily korban tomidim.

The Ben Yehoyada suggests that if we accept the latter view we can understand why Maariv is "reshus" (optional), as held by R. Yeshosha (daf 27b) given that it represents the "aivorim v'prodim" rather than the morning or afternoon korban tomid. By contrast, if it was the Avos who established the tefillohs, why should Maariv which Chazal tell us was established by Yaakov, have a different and lower status than Shachris (Avraham) or Mincha (Yitzhok), particularly when Yaakov was the "b'hir sh'b'Avos"?

His answer is based on the verse cited in the Gemara as the source text for Yaakov's establishing the tefilloh of Maariv (Bereishis 28:11) "and he slept there, when the sun set."  Chazal tell us that the sun set early. In consequence, although Yaakov davened Maariv at "night", it was in reality still day, which is the time of Mincha. Note that Tosfos on the daf hold (DH: Yaakov Taken Tefillas Arivis)  that "and our minhag is attractive... to daven Maariv while it is still just day."

In consequence, Maariv is qualatively different from Shachris and Mincha in that firstly it to some degree duplicates an existing prayer, i.e. Mincha; and/or, in a more radical interpretation, if davening at night is not really the optimal time the prayer, then if we do so it is a “left over” tefilloh, bearing the same relationship to Shachris and Mincha that the aivorim v’prodim bear to the Tomidim.

For this reason, even if we hold that the tefillohs were established by the Avos, Maariv is considered a "reshus".

The Ben Yehoyada also points out that the Zohar names three angels, Michoel, Gavriel and Nuriel, as the angelic powers representing respectively, Shachris, Mincha and Maariv. The acronym for these angels is MoGaiN, a shield, which Dovid haMelech hints at when he says (Shmuel 2:22) "and give me the shield of your salvation." Prayer is a shield.