Tzaddikim explain that taking three steps forward [before Shemona Esrei] is to remind the person that he should come closer to Hashem when he wants to pray.

He shouldn’t approach tefillah with the same level of emunah and bitachon that he has throughout the day. He should come nearer to Hashem by increasing his emunah and bitachon, and only then he should begin to daven.  ~ R’ Elimelech Biderman Shlita