Question

why is 10th class so important ??

Answer 1

In this class we have CBSE BOARD

Answer 2

Class X is important for a number of reasons. 
As it is the first external exam you will appear and you will be assessed in these,not by your teachers, but by people who do not know you personally. Hence their assessment will be unbiased and will you a very good idea as to what are your strengths and weaknesses. 
Good performance in these exams will serve to enhance your self confidence which is necessary for you to stick to and perform well in your competitive exams after class XII. 
The results may also help you to decide which courses to follow for your +2. 
When you are job hunting in later life a good Class X results will help give you that much advantage over those who haven't fared as well as you. 
Nowadays at the time of placements most companies ask for Class IX, X, XI and XII mark sheets as well as those of your BE or other qualifying exams. 
Admission to Foreign Universities becomes easier as well with good results in the above classes.

Hope this helps!!!