What is the purpose of using your initials instead of your full name in oral presentations and when is it appropriate I noticed during some conference talks that some presenters use their initials rather than their full names on some presentationn slides. Specifically I noticed this when they show name-labeled photos of their teams or have slides where they highlight key contributors to a broader scientific field.

I'm not sure whether this is a cultural quirk (I did't catch any pattern regarding nationality), a means to draw attention towards your research team (it seems to be more common for team leaders and professors) or just a boastful way of understatement (since everybody knows who "X.X." is).

Additionally, is  doing this appropriate and/or advisable for an (under)grad student and if so, in what context?
The convention in mathematics is to use your last initial during talks when citing your own work. Probably it seems a little vainglorious as well as unnecessary to write out your entire name. There's no reason undergraduate/graduate students shouldn't participate in the same conventions as more experienced researchers.