"What are you wearing that thing for?"
"It is the standard outfit for a priest."
"Is it required?"
This conversation between a parishioner and myself occurred years ago before my current assignments. I knew that they were not used to it, but the "norm" for Catholic priests still remains the cassock. I know that the USCCB got special permission to allow the "black suit"; there is an issue with this "new norm" though. The "black suit", though it is recognizable for many once they are close up, there are many more who do not see it as anything other than a strange outfit. I cannot tell you how many times people have seen me in the "black suit" and said, "is that some kind of special uniform?" It does not connote "Catholic priest" the way that it used to.
So then, no, it is not required, but the Church's Directory for Priests says that we should be wearing something that is noticeably different from the rest of society. This is for the purpose of being able to be seen when needed; the presence of the man of God in the community is important. He should stand out because when someone realizes we have not gone away, it impacts their understanding of the Church ("yes, we're still here!"). Just a few weeks ago I was stopped by someone in the store and told "I didn't know there was a Catholic Church around here; it is good to see a priest wearing his clericals." Just being in the store said that I had a Church nearby, and it made the individual (who was not actually Catholic) realize that the town has a Catholic presence.
Aside from the fact that the "suit" does not stand out nearly as well as the cassock, there is another aspect that I have to mention. The cassock is a bit uncomfortable--and that is a good thing. It is good because it reminds me that I am not running through life to make myself happy, but to serve others (and doing so is often uncomfortable!). It is like the collar itself (which carries the symbolism of a slave's shackles) personally telling me: you are not your own. In our modern age, we have been pampered beyond some of the wildest dreams of our forefathers; and we are constantly demanding more. The priest should never give in to this because he must be an example to his flock.
The Directory mentioned earlier even says that if a priest refuses to wear his clerical garb, then it could very likely be a sign that he does not understand what it means to be a priest. So many priests have been influenced by bad practices, and even worse theology that many of them today think that they are supposed to be "one of the guys" and avoid any sense of being different from the laity. This is truly a sad state of affairs. If a priest really wanted to be "one of the guys" then he should not have been ordained as a priest.
The more that society around us loses its touch with reality, the less it understands about the Church. Theology becomes personal opinions and the Church becomes a support group. In this context, we must go out of our way to make sure that we are being clear about who we are and what we believe. This is a part of evangelism, for if the world thinks that we are just like them, and we do nothing about it, then we are, essentially, complicit in their errors. Will this mean that we will have to suffer for it, as so many of our forefathers have done before us? Likely, yes. But that cannot deter us. I have gotten stares and even an occasional bad comment, but for the ones who come up and say, "you're a Catholic priest, right?", it is all worth it.