According to his editor, DFW thought that everything in the novel was necessary for the work as a whole. Post-modernism operates partially in response to stream-of-consciousness modernist lit, wherein the authors (Joyce et al) admittedly vomited onto a page in order to authentically capture, without editing, the essential element of literature, the authorship and the artist's mind-gaze, so I would hesitate at thinking it is just a flurry of homeless ideas. Also, he admitted to using the footnotes as a way to disrupt the narrative and any linearity you even have. Confusion is only natural and this is part of the experience of reading the novel. You will not gain much by following just the plot while reading. The joy is simply reading the work and the thoughts pouring out.

There's a world of difference between post-modern philosophy and post-modern literature, but DFW was aware of both as the son of a philosopher and he incorporates some hard-line philosophy into his work, however, post-modernism in literature is also (partly) about the relationship of the author to the work, the reader, the voice, and other literary forebears. This is not to say these things were not taken into account beforehand, but previously was not so endemic in the writing.

The "asides" and plots tangential to the main thread is an stylistically classic example of post-modern literature. Italian Neorealism also contained some concurrent stylistic choices (though obviously the gaze is literally controlled by a director, so there are essential differences, but this video helps visually elaborate similar stylistic choices), namely a focus on the world inhabited by the characters and not simply treating them as background actors (or "figurants", DFW ruminates on this at a later point in the novel. the relationship of the characters to the main plot is very very important, and pay attention to that. Not necessarily what they are doing in the plot, but their relationship to the plot. ie, how many of them make choices, what choices does anyone make, are any of them drone-like in response to the robust environment around them? are you even watching the main characters of the actual story?)

The term for such a splayed-out world is an Encyclopedic Novel. The ability to not just regurgitate common narratives, but instead investigate their origins and the fractionation amongst the belief systems, and where the knowledge to build these ideologies comes from is central to this stylistic choice (he does this a lot with his elaborations on trope-y sayings ("One day at a a time") the context of their meaning and assmiliation). It is not a whimsical one done idly. This is a driving force amongst post-modern lit to discuss our relationship with knowledge and information, which I believe to be an important aspect considering we now live in such a fractalized and information-rich society.

In video games, I would say that that is less of an meaningful choice to establish an idea in the narrative, but to generate further environmental and emotional investment in the plot and characters. Not to say that they are not indebted to literary and film influence, but I don't think they have sufficiently matured to the point at which you can say either "This is an art form that encompasses all possibilities of the form" or "they contribute to the understanding of their place in society"

It's difficult to try and parse out exactly "Why" each character and plot is weaved in, but they have some significance and meaning, and a lot of times these digressions are used to reinforce and elaborate on particular themes (of which there are a lot in IJ), to provide counterpoints, or different interpretations.

Don't sweat too much trying to tease apart a plot, you will miss half of the real reason why this novel is so enjoyable and thought-provoking, and going back afterwards and finding a timeline of chronology is easy to (do not do this while reading). Really think about who's eyes you are looking through and what contains meaning in all the information. DFW did not just put the puzzle pieces on a table and say "figure it out", post-modernism is looking at the pieces, realizing most of the shapes don't even match or are redundant, then wondering about the economic system that brought the puzzle to the store you bought it from, what the worker's lives are like who made the thing, and if they've even know what or will ever see for themselves the Eiffel Tower on the front of the box.