G-d promised in the Torah that Moshiach would come. If so, why is there a need to pray and demand his coming?
Prayers for the Redemption take up a large percentage of our daily prayers. The eighteen blessings of the amidah (silent prayer of requests that Jews recite three times a day) includes no less than six blessing beseeching G-d to gather in the exiles, return us to Zion, and restore the Davidic monarchy. Why are these prayers necessary if G-d assured us in the Torah that He would send the redeemer?
Maimonides defines the mitzvah of prayer thus: "One should request [of G-d] all of his needs." In other words, when we are in need of something, we should turn to G-d and ask Him to grant it to us. If this is true of personal needs such as health and livelihood, how much more so for needs that are both personal and communal.
Despite the fact that we are confident that we will eventually be redeemed, we cannot sit back passively and wait for the Redemption to come "tomorrow." We must be persistent in our demand that Moshiach should come immediately.
When we simply wait passively, we are indicating that the Redemption is not really a primary goal or need of ours. It indicates apathy – we are satisfied with life in exile and don't feel a need for change. Requesting and demanding the Redemption sends a clear message that exile is not satisfactory for us – the Redemption is a necessity.
There is no doubt that requesting Moshiach's coming in our prayers can hasten his arrival – sooner than he would have arrived without our prayers. For this reason, our sages instituted the prayers for Redemption as part of the daily liturgy.