In general, it is clean for me.
really? 
just kidding... script is a fundamental part of the way, the bitcoin system works.Quote
But i can not understand for what transaction scripts are needed?
next to the links already provided (and I really recommend reading Andreas' book "Mastering Bitcoin", these scrpts make sure, that only the defined target user (or owner of that address) can spend a transaction.Quote
Why was this introduced?
Difficult to say, but based on previous developments, one had to make sure, that in a P2P world, there might be malicious actors. So one had to make sure, that you can not spam the network, you cannot double spend a transaction, you cannot create values out of thin air... all this has to do with the scripting. Quote
It seems for any transaction the script is same, so , why it is used here?
no, definetly not!!! There are two parts in a transaction, which conatin scripts. In the input section, and in the output section.
The input section contains the condition, which must be met, so the funds can be transferred from the previous address to a new address.
The output section contains a script, that proves, that "I" had the correct keys, to be able to move the funds.
This all has to do with cryptographics (especially hashing and signing, you will step over the words of ECDSA...).
If interested further, the links above explain alot, and here is two links, explaining the signing process and more:
http://www.righto.com/2014/02/bitcoins-hard-way-using-raw-bitcoin.html
https://bitcoin.stackexchange.com/questions/3374/how-to-redeem-a-basic-tx