I mean Great Vespers...Sorry misunderstood your original question, the entrance is a hold over from the Cathedral Rite Vespers, it is when the Bishop would enter the altar area at that point. Attend a Vesperal Liturgy with a Bishop serving and it will make sense.
I know what happens. I've been to Great Vespers many times. I meant, why is there an entrance? The historical reason behind the great entrance during the Divine Liturgy is that the unconsecrated elements used to be housed in a separate room, and the entrance was when they were brought to the altar. So what's the historical reason for the Great Vespers entrance?
Like the origin of the other entrances where the deacon was sent to get something, (the Gospel Book and the gifts at Litrugy) and bring it into the Church, at Cathedral Vespers he was sent to get the lamp.
"...the cathedral of Hagia Sophia in Constantinople contiued to retain the ancient usage of bringing in a lamp from outside the church...The service began with the huge cathedral in complete darkness. At the time appointed for for bringing in the lamp, the archdeacon took the censer and, accompanied by a second deacon along with twelve other deacons, processed from the sanctuary to the narthex, where burning lamps were already prepared and waiting. The deacons took up the lamps and the archdeaconled them back into the nave of the church. The archdeacon intoned, "The Light of Christ illumes all men," and ascended the ambo with the other deacons. Then the cleric designated by the rubrics as the "superior of the lights" likewise took up a lamp and approaching the patriarch in the sancturay, said: "Bless, master." The patriarch answered: "For Thou art our sanctification, O Christ our God, always, now and ever, and unto ages of ages." The superior of the lights then lit all the lamps in the sanctuary; the subdeacons and lamplighters did the same throughout the church. After the deacons standing on the ambo received the blessing of the patriarch, the archdeacon intoned, "Wisdom, let us attend," and they all descended the ambo and entered the sacntuary"(Uspensky Pages 32-33 Evening Worship in the Orthodox Church