What is the unique significance of wine that we use it for the arba kosos - the four cups of wine? Indeed, why do we use wine for kiddush every Shabbos and Yom Tov?
Rav Yakov of Izbitz (ספר הזמנים על הגש"פ) explains that wine is the epitome of physicality – הטובה המובחרת בעוה"ז. This is why wine causes drunkenness, for the nature of the physical world is to confuse the mind and reduce our spiritual sensitivity. Therefore, on the Shabbos we specifically make kiddush – “sanctification” – on wine to indicate that all our physical activities and pleasures throughout the week were done for the ultimate goal of serving God.
This concept has extra significance on Pesach. God freed us from Egypt so that we should serve Him. (ראה בספר חרדים ט:כד באריכות) As long as we were slaves in Egypt, we were unable to be fully devoted to God's service, both physically and spiritually, for their physical actions remained under the control of their human masters. Only בני חורין – free men – can fully devote themselves to the service of God. This is the symbolism of the four cups of wine, to express our commitment to channel, not only our hearts and minds, but every aspect of our freedom, even the most physical, to the service of God.