Why use combined characters that have the same meaning? (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.

If you would like to read more about how Chinese has developed two-syllable words, you can have a look at this academic paper, which was linked in this answer.