In the comments, jlawler linked an excellent discussion of definiteness. The short version is that all languages do something to mark information status, and a definite article is one way of accomplishing this.
So I'm going to focus on another part of your question: "If it adds expressivity but communication can be fine without it, why does language force only some people to always specify a certain category of linguistic meaning?"
The answer is, it adds another level of redundancy. Redundancy is crucial for spoken language, because speech is a very noisy channel and it's a certainty rather than a possibility that some information will get lost. So languages seem to optimize for a certain level of redundancy that conveys the optimal amount of information per unit of time—too much redundancy and you'll waste time saying too much, too little redundancy and you'll waste time going back and re-expressing things that got lost.
Mandatory marking helps with this in a few different ways. It can duplicate information found somewhere else: for example, English indefinite articles have redundant number marking ("a" always has to be singular), and German definite articles have redundant case* and gender marking. This helps if some information about the noun is lost; if you lost a syllable from the middle of a German noun, for example, the article tells you its gender and lets you narrow down the possibilities.
Also, marking tends to be mandatory for specific categories of word. In English, for example, articles and plural marking only go on nouns. This gives a different sort of redundancy; if some information about a word is lost, you can still figure out what category of word it was. This can also help with some types of ambiguity: you can tell the difference between "a record" (noun) and "recorded" (verb) immediately because of the types of marking they show.
But, what exactly is mandatory to mark can vary a lot. Languages generally benefit, in terms of redundancy, from having some trait marked on some category. But different languages can, and do, choose very different traits on very different categories to mark. If one benefit of mandatory tense marking is to show which words are verbs, mandatory evidentiality marking would also serve that purpose just as well. Different languages have found different solutions to the optimization problem and reflect it in different ways.
* Even as case marking on nouns fades, it's still often conveyed by e.g. word order and choice of prepositions. The article isn't the only way to know which noun in a German sentence is the subject, for example.