There are reasons, however, that such things are and have for quite some time been fairly standard parts of collegiate curricula in the Western world (however fraught such terms as "standard" and "Western world" can be). One fairly instrumental reason that I tell my students and that I try to demonstrate to them through example is that there may well be circumstances, regardless of a person's field of study, that require such things. Anyone may be called upon to give testimony in front of a judge, a jury, and lawyers, for example, and many people will have to make a solid argument for getting money--venture capital and initial investments, after all, do not come with the rain. Practicing how to make such things happen then becomes potentially important.
Perhaps more important is that the students themselves are subjected to many such appeals. They, and I, are inundated with arguments each day. People try to get us all to do things, to give them money, to adhere to certain codes of behavior and standards of conduct. They work upon us, and I work upon my students, and the students work upon me, to produce specific results, not all of which are to the benefit of those upon whom the work is done. It is therefore necessary for all of us to understand how arguments are made so as to be able to perceive when others are trying to take advantage of us. There is no way to offer meaningful resistance to manipulation otherwise, and I do not know many who delight in being manipulated. Conversely, when we see that others are at least making an honest and sincere attempt to persuade us appropriately, we can see that they are at least according us some measure of respect, and we can therefore know better how we ought to treat them.
Related to unpacking how others attempt to manipulate or persuade us is unpacking what our utterances and those of others reveal about us. The things to which we refer indicate much about what we value; they show what we think is important and what we think our audiences think is important. If I say that something is a Sisyphean task, I am showing that I take as a common reference point certain aspects of Classical myth--or that I think my audience does. (Such things also allow for an economy of speech and writing--I can say the task is "Sisyphean" rather than having to explain that it is one which I believe to be "fundamentally impossible and futile to pursue but equally impossible to set aside." Many of my students express a desire to "get in and get out," yet they despise trying to master sets of knowledge that allow for such referentiality, and I do not understand it.) They speak to our acculturation and our ability to understand and engage with the cultures in which others are enmeshed, promoting understanding of ourselves and one another. In such understanding, we can come to better know one another and to realize the common humanity of which we all partake.
It is an unfortunate truth that many of the problems we have--the various "isms" that are so frequently decried come to mind--are a result of people not understanding one another. Many other problems result from people not valuing one another, however well they may or may not understand each other--although I tend to think that the undervaluation proceeds from an incomplete understanding. It follows that developing a superior understanding will help to solve those problems, and since study of the humanities helps to foster that understanding, it can help to improve how we interact with one another and, it can be hoped, the world in which we do so.