Capitalisation is the sort of thing you stop noticing once it is right and cannot stop noticing once it is wrong. A heading in the wrong case looks broken even to people who could not tell you what the rule was, and it is one of the fastest signals a reader uses to judge whether a page was made carefully.
The three cases worth having
Sentence case capitalises the first word of each sentence and leaves the rest alone. That is how ordinary writing works and how most style guides want body text written. Title Case capitalises the significant words in a heading or title. And uppercase is for the labels, the acronyms and the places where the convention genuinely is capitals.
Having all three in one place removes the guesswork from switching between them, which is the actual task. You are rarely converting one piece of text once; you are moving between a heading and a paragraph and back, and doing it by hand is where the inconsistencies creep in.
Where the awkward cases live
The difficult part of automatic capitalisation is not the first letter, it is the decisions. A title containing a colon usually capitalises the word after it. A word following a full stop inside an abbreviation should not be capitalised, and naive tools get this wrong. Hyphenated compounds, names with particles, and anything containing an acronym or a product name all need judgement the tool does not have.
What this does reliably is handle the bulk consistently, so the exceptions stand out rather than hiding among a page of inconsistent capitalisation.
Why not the shortcut
Shift plus three capitalises the current word, which is fine for a title. It does not handle a whole document, it behaves unexpectedly when a selection spans sentences, and in several common shortcuts it lowercases the rest of the line, quietly changing text you did not mean to touch. On a short heading that is harmless. On a body of work it produces exactly the kind of inconsistency you were trying to remove.
Where to use it
Headings, page titles, captions, navigation labels and any metadata field with a length limit. If you maintain a site or write regularly, doing this consistently is a small habit with an outsized effect on how finished the result looks.
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Case in headings and metadata
Beyond body text, capitalisation matters most in the places a search engine reads rather than a person. A page title is the single most important string on a page, and it should be sentence case with the important words capitalised, because that is both the convention and what search results display most predictably. Meta descriptions are usually written as a sentence, with one capital at the start.
Navigation labels and button text take sentence case as well. Title Case on a short interface label is one of the fastest ways a polished product starts to look amateur, largely because it is associated with older software rather than current design.
Acronyms are the exception worth protecting. An acronym like API, URL or PDF should stay in capitals whatever case you apply to the surrounding sentence, because converting it changes what the word is. Most tools will treat them as ordinary words, which is the one case worth checking by hand.