Why testing is necessary?

Before start learning of Testing i would like to ask one thing - why testing is necessary?

In each and every field 'Testing' is the important phase of product development cycle. Without testing no one ensure the quality of the product. The product can be anything it may be a Car, a Machine, a Medicine , a Software etc, it need to fully tested. Only after testing you can say the product that you build have good quality. Only after testing you can release your product in market. if you release your product in market without testing then it may be possible that your product have serious issue that cause the negative effect on your business. So Testing on important.

Now we talk about the Testing importance in Software Development.
Testing is necessary because everyone makes mistakes and sometimes these errors are expensive or dangerous. Testing is required to test every part because things can always go wrong.
Today is the market of Internet and if you leave any mistake in your software, user may be switch to the other product due to there are lots of competitors today, so we should have close eye on our product and check it very carefully so that we can minimize the mistake and provide the quality product.
Whenever any user perform any work , then he is pretty sure that he/she have done some mistake. when he/she itself try to find the issue, he/she unable to find it because he/she follow the same steps again and again. So he/she need to other one help with fresh mind. Another user use new steps to follow the procedure because this user absolutely new for this product . He/she follow the new steps then he/she can able to find the issue that first user can't find. This is called testing.

Feel free to ask anything.