arguably it makes what would otherwise be a very little letter stand out more in text

I agree. A quick search tells me the practice was started for legibility in hand-written manuscripts [1], or for that and also to emphasize the importance of the writer (seems egotistical to me :)) [2], or those reasons and also to help distinguish English from other languages but we don't really know [3].

[1] http://www.alt-usage-english.org/excerpts/fxwhyisi.html [2] https://www.thesaurus.com/e/grammar/whycapitali/ [3] https://greatbigstory.com/why-do-we-capitalize-the-word-i/