The Mord'chai on B'rachos (#19) implies that the reason for the practice is to familiarize oneself with the words of the Torah. Ramifications of this law include the automatic fulfilment of it by teachers of Torah to children and the fact that a commentary is a valid substitute for the translation. Others disagree, holding that there is special significance to Unkelus's translation and that meanings absent from the commentaries are conferred by it (See Tosafos on 8a, s.v. " שנים מקרא ואחד תרגום".).
That this practice should enable one more than simple scheduled familiarity and actually preparation for the weekly public reading is implied by the Shibolei Haleket (#75) when it specifies that one should finish his reading before heading to shul in the morning. (Although it does state that failing to do so, one should still read it before eating anything later that day. Furthermore, Kol Bo (#37) even opines that in such circumstances one may read up until Wednesday.) It also may be the implication of Rambam in Hilchos T'fila Un'si'as Kapayim, 13:25, when he disqualifies hearing the weekly public reading from being a fulfilment of the obligation.
A very interesting limitation to this practice is made in a responsum (#88) of the Ra'ava"n when he explains that sh'nayim mikra v'echad targum applies (or was originally intended) only for individuals who live in secluded cities without a minyan. They are to emulate a fulfilment of public reading by reading once for each of the two people reading directly out of the Torah and once for the real-time translator who it was the custom to follow along for public comprehension at the exact time it was being read elsewhere in public.