For example, belan virinon (beautiful woman, in case I'm being unintelligible) or belaj virinoj (beautiful women). Imagine there is more sentence there. Why does the adjective share the ending of the noun (object and plural used as examples here)?
Vere, kial ne?
Mi supozas, ke tiun decidon faris Zamenhof kaj mi ne scias ĉu li respondis al ĝi en iu de liaj lingvaj respondoj, sed se mi devus supozi, mi dirus, ke unu el la kialoj estus tio, ke multe da lingcoj faras tion:
- Slavaj lingvoj faras tion;
- Latinidaj lingvoj faras tion;
- La germana lingvo faras tion (mi ne scias ĉu la aliaj ĝermanaj lingvoj faras tion).
En ĉiuj tiuj lingvoj, la adjektivoj konkordas kun la substantivoj ne nur en la singulara kaj la plurala formoj, sed ankaŭ kun la genro de la substantivoj. Tamen, Esperanto ne havas genrojn.
La evidenta escepto estas la angla lingvo…
Really, why not?
I'm guessing that was a choice that Zamenhof made and I do not know if he has answered it in any of his lingvaj respondoj, but if I had to guess, one of the reasons would be that many languages do that:
- Slavic languages do it
- Romance languages do it
- German does it (I do not know about the other Germanic languages)
In all those languages it's not just the singular and plural forms that agree, but adjectives also agree with the gender of the noun. Esperanto doesn't have genders, though.
The obvious exception to the above list is the English language…
As mentioned in Lyubomir’s answer, presumably the reason is just because that’s how it’s done in many other languages. However I’d add to that that the adjective doesn’t always match the noun, it matches the thing it’s describing which can be more than the following noun. In some cases this can resolve some ambiguity which can’t be done in English. For example:
Kio estas blasfemo por unuj, estas religiaj penso kaj emocio por aliaj.
Here “religiaj” is describing both “penso” and “emocio” so it is plural, even though the words it is describing are singular. If this was “religious thought and emotion” in English, it would be ambiguous whether the emotion is religious or not.
Adjectives agree with nouns in case and number (that is, they share the -j and-n endings) because the rules of Esperanto require it. Not making your nouns and adjectives agree would mark a speaker as a beginner and call attention away from what they're saying and onto how they are saying it.
Many of the languages that Zamenhof (the first author of Esperanto) was familiar with included adjective agreement, and so it would have been a natural thing to consider including in the language. Esperanto's system of agreement is much simpler than that of Latin, Russian, German, and even Spanish or French.
Adjective agreement is a form of linguistic redundancy. That is, it means that it helps to ensure that a bit of information (case or number) is included more than once. This aids communication in situations where the communication environment is noisy, the listener is distracted, or the speaker mumbles or makes a typographical error.
Adjective agreement allows for more flexibility in word order and makes it clear which adjectives go with which nouns. This gives experienced speakers more options in how to express things in ways which are more difficult in languages which don't have agreement. It's also often claimed that this gives beginners with different linguistic backgrounds the flexibility to use a word order that comes naturally to them.
This is the general way inflecting languages work: Inflection is there to create coherence and redundancy, not only a meaningless decoration. So even in the very mildly inflecting language Esperanto, there is so-called agreement between the adjective and the noun: They share their inflectional endings.
Note, that you can use it as a creative device: The adjective does no longer need to be a direct neighbour of the modified noun, e.g.,
Belaj estas la virinoj en Parizo.
It eliminates ambiguity. Humor is often based on ambiguity, but is deadly in various other venues, especially travel.
To illustrate, there is a famous joke from the movie “Mary Poppins” which would not work in Esperanto. It goes like this:
A: “I know a man with a wooden leg named ‘Smith’.”
B: “Really? What’s the name of his other leg?”
The humor depends on the fact that English does not distinguish between “Mi konas viron kun ligna kruro nomitan ‘Smith’.” (the intended meaning) and “Mi konas viron kun ligna kruro nomita ‘Smith’.” (the intentionally-misinterpreted meaining).
With Esperanto, we lose this item of humor, but on the upside, two airliners avoid colliding with one another.
The others mentioned legacy from other languages that Zamenhof spoke. I'd like to point out and clarify, that just like the other elements of the language, this one is useful. It makes it possible to experiment with the word order without causing ambiguity.
Look at these examples:
- La viro altan domon vidas.
La viro alta domon vidas.
Knabino sercxas hundon kurantan.
Kanbino sercxas hundon kuranta.
Sxi estas kun amikoj honestaj.
- Sxi estas kun amikoj honesta.
The sentences in a pair has different meanings.
Why would one change the word order like this? Not put the adjective right before the noun or pronoun it describes? Firstly, it can be used for emphasis. Secondly, it is very liberating in song-writing as well as poetry, where the flow and sound of the words matter, not just the meaning.
Tio okazas ankaux en la franca kaj hispana lingvoj.
Mi ne tajpis "la francaj kaj hispanaj lingvoj", pro tio ke mi skribis pri nur unu franca kaj unu hispana. Do tio ne cxiam okazas. Kaj gxuste cxi tie vi vidas la utilon.
Mi ne igos mian respondon tro longa.
En tiu frazo, "respondon" finigxas per "n" kaj "longa" ne. Cxio cxi estas iom pli komplika ol gxi eble unue sxajnas.