Can I just point out that the use of a combination of both have a long history? They are half-uncials)
They were rather common in days of yore. You can see them for example in these kind of fonts:
I know of places where people write capital R in a regular handwriting, otherwise consisting of lowercase. This I found in Ireland particularly, and maybe that has something to do with a long history of Irish half-uncials? (maybe @Jenna can shed some light on this?)
When used in the olden days, I assume it was saving line-heights (vellum and paper wildly expensive).
Imagine that the development of letters went from uppercase to lowercase-ish script (faster writing) cursive. There is also the consideration that larger variation in lettershapes in tiny text might make it easier to read. Lowercase n m u, d a etc. can be very similar.
To differ lower and uppercase was not really important.
Today it might be seen as "childish" and "playful" and might be used for brand-buidling as there is a subtle difference in using a lowercase in an uppercase text. It is a simple subtle difference.
Personally, I think we should play more with this. The "rules" of upper- and lowercase deserves to be shaken a little.
And here is an interesting evolution of this:
(from this Wikipedia article)