The human brain contains neurons that communicate with each other in order to achieve the evolutionary goals of survival and reproduction on an ongoing basis, which is a result of the optimizing aspect of the evolutionary process.
Such goals are achieved by utilizing knowledge and information gained from past experiences and applying it to the dynamically changing current interactions and making future projections.
The optimization process requires such knowledge to be shared between people, based on the adage “knowledge multiplies when shared”. When you share knowledge, it increases in multiple ways. The overall knowledge amongst people you share knowledge with increases, as all of them gain the knowledge about what you have shared.
Once the knowledge is shared with others, they can apply, test, build, refine and expand upon it (and vice versa), sharing which expands not only their knowledge, but your knowledge too.
The currency to gain and utilize knowledge in the brain is the thought process. As your thoughts are limited to the host brain, the optimization process has resulted into development of a code, i.e. language, which can be used as a system for communication between people.
The following analogy explains why language is required for people to communicate with each other (using the DOS model):
Factory Analogy for Language
Companies from different countries have their factories in an industrial zone. Owners of all these factories only speak their native language, and thus, cannot communicate with each other.
Communication between them is important as it can help them maintain relations and exchange knowledge, experiences, resources, etc. while also helping to make effective implementation of rules and regulations designed for the overall growth of the industrial zone.
To make such communication possible, each company hires a person from their native country as a manager who learns a code which is developed specifically for the purpose of such communication.
In the analogy, each company represents a person, the owner is LB, the manager is VP, native language is neural communication, industrial zone is a group of people, the communication code developed for their communication is language, e.g. English, and learning the communication code is learning the language.
As per the analogy, LB is only capable of neural communication (i.e. intramural communication), whereas VP is capable of neural as well as verbal communications, i.e., extramural communication. Putting it differently, VP is the interpreter between neural and verbal communications.
Note: The neural communication mentioned here is limited to interaction processing in the human brain (more under title Interaction Processing System (IPS).
By default, our brains process our ongoing experiences by recalling our past experiences and knowledge that is directly and contextually related to them (more here). The same is with animals and other organisms.
In order to further optimize the decision making process of our ongoing experiences, our brains need to deliberate, contemplate, cogitate, speculate, etc. to plan our current and future actions, which is not possible, as memories cannot be recalled at will.
To recall memories at will, we humans use language.
How does language work?
Contents of our experiences, i.e. characteristics or particulars of a person, thing, group, idea, class, occasion, etc. (including subjective experiences) are tagged (or identified when already tagged) with concept names and stored in the memory.
Such association by tagging makes it possible to recall such memorised contents by conjuring up or receiving their associated concept names and vice versa - i.e. recalling concept names on the occurrence of or interaction with their associated characteristics/particulars.
Concepts are stored in hierarchical manner and linked by their contextual facts, events, perceptions, sensory experiences, emotions, feelings, etc. so they can be quickly and easily recalled using language in direct and contextual manner as and when required (at will) by navigating such hierarchies.
By using concepts through language, we can directly or contextually 1) bring up multiple facts and aspects of contents of our experiences, which enrich our thought processes and knowledge by enabling us to contemplate, deliberate, ponder, etc., thus enabling us to plan our ongoing and future experiences in optimized manner and 2) pass on benefits of the same to others, while expecting same from others and so on, making it the chief contributing factor responsible for the development of human intelligence, only limited by how developed the languages are and how they are utilized.
In a nutshell: The dual benefits of using language are that it 1) helps us recall and manage memories, resulting into enriching our thoughts and 2) enables brains to communicate with each other by exchanging such thoughts.
Nonhuman Animals and Organisms
As nonhuman animals and organisms do not use concept processing through language, they cannot recall specific past experiences and knowledge tied up with them at will to plan their ongoing experiences and future experiences.
Their brain can only recall memories and knowledge that are similar and/or contextually related to their ongoing experiences (more here).
Even with such limitations, they demonstrate fairly intelligent activity and behaviour. Such intelligent activity/behaviour is a result of hard-wired, repetitive and condition based interactions or combination of such, which are instigated by the goals of survival and reproduction and/or human interactions.