As someone linked to the humanities as well as applied sciences, there are a few reasons for reading a paper verbatim at conferences.

 1. Time constraints: humanities scholars are famous for
    rambling endlessly, losing track of the key point, and going way
    beyond their allotted time. Therefore it has become more and more
    common to see reputed conferences demanding their presenters to
    submit beforehand to the session chairs not just the paper draft,
    but even the presentation slides as well (if they exist). These are
    then reviewed by the session chair(s) who will recommend edits and
    changes. Unsurprisingly, most of the proposed changes are for
    cutting down parts of the text and to tighten up things, in order to
    make sure that the spoken text can fit within the allotted time.
    Despite all of these strict precautions, there are still a few cases
    where the presenters completely lose the thread during the
    presentation, and start rambling, only to be unceremoniously shut
    down at the end of their time.
 2. Difficult fields such as Philosophy and Religious Studies require a
    very high level of precision in regards to the terminology employed, the definition of terms, and the
    method for exposing one's arguments. Otherwise, it will be very
    difficult for the audience to adequately apprehend the topic and its
    many nuances, or provide any meaningful discussion or debate around
    it. For example, on an average Humanities presentation the speaker is often required to quote numerous textual passages without any mistakes, and do what is called a "close reading" of those passages, which requires the use of very precise language. These two things alone tend to push people into reading texts, rather than talking more spontaneously. It's already challenging enough to talk about things such as
    Ontology, Eschatology or Esoteric Buddhism, now try doing it on a
    strict 20-minute limit, without any text to guide you along the way.
    By reading a text verbatim, the speaker can have enough time to hone
    the text beforehand, and make it as clear and well-structured as
    possible.
 3. It has become more and more common to conduct conferences (both
    online and offline) that include speakers talking in different
    languages, thus requiring the temporary hiring of translators.
    Whenever translators are involved in a conference, having a prepared
    text can go a long way to reduce costs and make sure that the
    quality of the translation is at its best. This also makes conferences more inclusive for speakers and attendees coming from non-Anglophone regions.

Now, having said this, a skilled speaker will normally do various practice sessions in order to become familiar with the text, and allow for making direct eye contact with the audience while still reading from the text. Personally, I normally practice at least 5 times in order to test everything thoroughly: the flow of the spoken text, the flow of the slides and overall structure of the slides, etc. I also designate key moments where I stop looking at the text and point to key aspects of the slides with a mouse or a laser pointer. All of this preparation time pays off handsomely, as it helps to make the reading text feel much more natural, and to better engage with the audience. I often tend to get better response from attendees after the presentation is over, which leads to being more successful at networking.

By the way, here is a funny point: if you see someone who is "apparently" talking about a topic freely in a skillful manner, it's more likely than not that they are reading from a pre-written text, although they are able to disguise it by adding several short remarks that give it the impression of being more "natural". All of those TED presenters are actually reading from teleprompts, but they had to rehearse the whole thing 20 times or more, so it's easier to "disguise" the fact that they are reading a carefully-prepared text.  

EDIT: Forgot to add one more reason. It is often the case that a presentation at a humanities conference is the first step towards writing a full paper and submitting it to a good journal. Since draft papers for popular conferences must be submitted and reviewed in detail by the session chairs, this provides a precious opportunity for getting valuable feedback from experts. I have often seen actual papers published in reputed journals, where the "Acknowledgements" section says something like this: "The contents that comprise this paper are significantly revised versions of two presentations made at Conference A and Conference B; I would like to thank the session chairs and the following attendees who provided valuable comments: (Person A); (Person B)"...