Isn't that obvious what we are talking about in the following sentences? If it is obvious, why we need an extra word in there?
The world is beautiful.
World is beautiful.
The pope said these words.
Pope said these words.
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No language needs any of its features, it's just the way it is because of how that language evolved throughout history.
In English, if someone asked you "Do you like cake?" you could reply with "Yes, I like cake". In Japanese, you could also reply with what literally translates to "Yes, I like cake".
However, you could say what literally means "like cake" or even just "like", and it would be perfectly correct Japanese. It would be incredibly strange if you said that in English. Even though I could figure out what you meant, it's still wrong, because that's just how it is in English.
Why do you have do add in the word "et" between a noun and a verb in Hebrew? Why can't you just say "I open the door", when you have to say "I open et the door", even though it's perfectly understandable? Why are nouns in German always gendered? Why does it matter whether I call a table "he" or "she"?
It's the same thing: because that's just the way that language is, because of the history of how it developed.
There is a lot of redundancy in natural languages, such as for example almost all forms of agreement (number, gender etc).
One of the consequences of this is that you can miss parts of what you're hearing and still understand it, with a lot of benefits for communication when it's noisy or people are distracted or making mistakes. The Wikipedia article gives a brief overview. Theoretical linguists (amongst many other things) can measure the amount of redundancy in given languages, speech acts and so on.
As for the specifics of why we need the definite article in some places and not others, here's a good article: https://davidappleyard.com/english/articles.htm
You might note that in abbreviated writing such as newspaper headlines, you will exactly find "Pope said ..." instead of "The Pope said ..."
But in ordinary speech, omission of an article normally introduces a proper name ("George said ..."), an abstract noun ("History says ...") or an uncountable noun ("Water is ..."). When we hear "the", we know that a previously-referenced (or ubiquitous), countable, noun is to be expected. When a listener hears "The X" and can't remember this X, the normal response is to ask "Which X".
Of course there are a great number of exceptions and special cases, but this is the general rule.
Len's and jonatanjo answers are good and insgightful, but I want to give you bit different perspective.
Actually, you do not use article with some well defined and understood thing, it's vice versa, you employ language tools to communicate and define the thing.
Let's start with the world and consider the examples:
The world is beautiful.
Given no previous relevant context existing, here the is used to specify that we are speaking about the universe, relying on the common convention.
Everyone loves these tiny worlds of plants.
Here we see that there are many "worlds" instead of single and very unique one.
The world of orchids is beautiful, but stinking lilies' one is very different. The world is ugly.
Here we use the same sentence to convey very different meaning.
Speaking about the Pope, I have impression that this is the same mechanics applied. Encountering in a text, "a Pope" I would expect the text to have some specific context like discussing one of the participants of imaginary "Pope day" when everybody is dressed like the Pope or just something completely different, as David Siegel suggested in his comment.