Thanks for your questions. I agree it is important to explain more about ecclesial vocations as a leavening agent that changes the entire Church, so thanks for the opportunity. First, we should say that all Christians are called to live the evangelical counsels in some sense. We do a disservice to every vocation if we see the counsels as only important for consecrated persons or only part of their vocations. At the same time, consecrated persons live the Evangelical Counsels in a radical way meant to serve as a Christic paradigm for others in the Church. Consecrated life serves the Church by reminding her of the Christ who is in her midst as Brother or Sister while also present as her Lord.
For priests, consecrated persons (and maybe especially religious priests) remind them that life in communion with Christ is profoundly prayerful and only that flows into service of equals among a communion of equals. This is important because it makes sure that the Catholic Church's hierarchical division into clerics and laity does not degenerate into a worldly thing and instead is genuinely Christian where the first become the last and the last become the first. The hierarchical nature of the Church is not problematical of itself so long as it continues to be, as much as possible, the hierarchy of the Kingdom of God. Should it degenerate due to ambition into a hierarchy of power and worldly status it is a greater tragedy than we can imagine. Consecrated women model the same Christlikeness of humility and service and do so (as do consecrated men) from positions of ministerial, communal, and theological expertise that challenge ministerial priests to always improve their own senses of these things so they may serve even better.
For the laity, consecrated men and women live lives dedicated to God in Christ and remind all the laity that this union must come first in every vocation. They have no money to give, but they invest in community and ministries that serve people in a myriad of ways, and of course, they give generously to the least and most needy in our society by pouring out their own lives for this purpose. In terms of the Church itself, consecrated men and women remind us all of the centrality of the poor Christ and the way we are each called to model him in our world. For the rest of the laity consecrated persons call them to aspire to more in their Christian lives. They provide a significantly countercultural model of success that is supremely loving and this summons both priests and laity to create both a Church and world marked in the same way.As John Paul II observed, a Church without consecrated persons and with sacred ministers and laity only, the Church would not be the Church her founder willed. Consecrated persons serve as a leavening agent that helps make sure the Church is countercultural, communal, and courageously and generously so. Hierarchy in this Body does not mean a privileged priestly class ruling over others who have nothing to bring to the Lord's table. It means a Body where all come around that table as a community made to serve one another with our own gifts, limits, and needs in whatever way we God calls us to. Again, consecrated persons are not a third level between the other two groups. Drawn from both priests and laity, they serve to summon all to an equality in Christ that allows the Church to truly image its Lord and his disciples in the world.
By way of answering your last question let me tell you a brief story. During the first years I had this blog someone write me about becoming a Catholic Hermit and wearing a habit. This person claimed to already have a habit she wore at home. She claimed to be able to pray better in a habit. I found this both pretentious and lacking in humility (loving honesty) -- two things that are antithetical to prayer. It is also theologically unsound. I can't count the number of times I have heard from those I meet that they believe religious have a special line to God, or that dressing up in a habit helps one pray better. Similarly, where serious prayer unfortunately becomes associated with those in a habit, so does ministry, the notion of union with God, or authentic holiness.