(rèn shí ) both characters means ( to know)and when combined it still have the same meaning. So why can't I use only one of them ?
Mandarin has a lot of two-character words, where both of the characters mean almost the same thing. This seems surprising, but makes sense when you see how the language developed over time. Old Chinese (spoken over two thousand years ago) had a lot more sounds than Modern Mandarin, and over time, many sounds were lost. This isn't obvious from Chinese characters (which don't record individual sounds), but this fact has been established based on ancient Chinese dictionaries (often called "rime books"), as well as modern linguistic research. However, after the sounds were lost, many syllables which used to sound different ended up sounding the same. To avoid confusion, people starting combining two syllables, to make themselves clear.
For example, rènshi (simplified: 认识, traditional: 認識) is composed of two syllables, both of which come from Old Chinese. They were pronounced something like "nəns" and "ək" (where "ə" is like the "a" in "comma"). We can compare this with other characters that are pronounced the same in Modern Mandarin. For example, 任 is also pronounced "rèn", but in Old Chinese, it was pronounced "nəms", not "nəns". Meanwhile, 十 is also pronounced "shí", but in Old Chinese, it was pronounced "gip", not "ək". By combining the two syllables, rènshi is easier to understand, because it can't be confused with other words. These two-character words have now been used for centuries, and in some cases, the individual characters would not be used on their own.
As a second example (from this question), we can look at shuìjiào (simplified: 睡觉, traditional: 睡覺). Both characters come from Old Chinese, and were pronounced "doih" and "krûks". In contrast, 税/稅 is also pronounced "shuì", but in Old Chinese, it was "lhots", not "doih". Similarly, 较/較 is also pronounced "jiào", but in Old Chinese, it was "krâuk", not "krûks".
I have taken these pronunciations from the ABC Etymological Dictionary of Old Chinese. If you want to search for other words, click "All" underneath "ABC Etymological Dictionary of Old Chinese", type your words in the box at the top (in traditional characters), and then click the "Display Phonetics" button on the left. If a character didn't exist in Old Chinese, it won't come up! For example, 苹果/蘋果 comes from Sanskrit, and came into Chinese around the same time as Buddhism came into China.
Sometimes they are used by themselves (it depends on the word), e.g.:
In my (only a learner) experience, the main reason seems to be that it's hard to understand sentences with single-character words:
认 and 任 are pronounced the same, and 识 is pronounced the same as 时, 实, and 十, so it's harder to deduce which character is being spoken.
认 and 识 also combine with other Chinese characters, e.g. 认为 and 知识, so it's harder to distinguish where word boundaries occur.
But 刚我吃饭 is not correct (because 刚 is an adverb). So if we arbitrarily interchange 刚 and 刚才, we will break the grammar.
Also, the English is only an approximate translation, and in Chinese these words that translate to the same thing are slightly different. They're the closest we have to the underlying meanings of 认识, 认, and 识.
It sources back to ancient times in China when spoken and written languages are in different forms (after Qin). Back then, only a small portion of Chinese people know how to write, while most Chinese people can only speak.
During the development of the Chinese language, written language became closer and closer to spoken language, transforming from WenYan（文言文） to BaiHua （白话）. In spoken language, people like to use bi-syllable words because it is easier to understand since there're many homophones. So written language inherit this pattern during its development.
In modern Chinese, people are used to this kind of expression, so it seems outdated or 'show off your WenYan skills' when you only use one character to express its meaning. Also, there're fixed rules that you must follow because of habits. However, you can still found some traces of ancient Chinese usage in Cantonese. I can't give an example here because I'm not an expert of Cantonese.
The evolution of BaiHua can be found on this Wikipedia page: https://en.wikipedia.org/wiki/Written_vernacular_Chinese
Please read this post: 认识 vs 知道 based on glyph origins
Both 认 and 识 has other meanings beside "recognize", just using 认 or just using 识 might mean something other than "recognize".
Put them together to form a specific term can remove ambiguity.
The reason for coining compound words is to turn general characters into specific words
For the same reason you say "come back" instead of just "back" - it's just how the language is.
Or, even more comically, you wouldn't say "pro" instead of "protrude" in English, why require such brevity from modern Chinese?
Modern Chinese likes bi-syllabic "words", if you want each character to be used for one separate "word", look into 文言文.
See Sections 5-6 of this paper for theories of how compounding and disyllabicization occurred together in Chinese. There is a long history.