Can you spot the differences with the messages above? The left side has a few more capital letters than the right side. Big O, little o. Who cares, right?
Well, if you write for an app or website, you should care. A little thing like capitalization can actually be a big deal. Capitalization affects readability, comprehension, and usability. It even impacts how people view your brand.
While there are some more objective arguments to be made, most arguments for and against either title case or sentence case mostly come down to whatever you're used to - what you grew up with. Title case looks entirely ridiculous and confusing to me, and makes dialog boxes, text, and other things much harder to read than when it's in sentence case.
The reason? We don't use title case in Dutch. Everything is sentence case. In English, it's mostly a case of preference, and either case type is fine as long as you're consistent.
Interestingly enough, Apple - generally considered the poster child for title case - actually localises its choice for case type. When you run Apple software in, say, Dutch - it doesn't use title case at all, opting for sentence case instead, because that's the norm in Dutch.
Title case also appears to be on its way out - generally, while pre-internet publications use title case, publications originating from the internet generally use sentence case. I wouldn't be surprised to see title case fall into disuse almost entirely over the coming decades in English - including at Apple. There's going to be an inflection point where title case will simply look incredibly out of place in English, as younger generations grow up on new publications that do not use it.
Title case is old - very old - probably because lowercase evolved out of uppercase, and over the centuries, we've been slowly pushing uppercase letters to perform very specific functions in text. Capitals have become an integral and core part of punctuation rules in every (?) language using on the Latin, Greek (?), and Cyrillic (?) scripts, and while there is some variation here and there - e.g. German holding on to capitalising every single noun, not just proper nouns - there's a remarkable consistency between them.
I'm fairly certain English' title case is the odd-one out, and as the internet continues to break down barriers between cultures and languages, title case will eventually disappear from English, too.