Before answering this question let me tell you that whatever you do in SSIS or SQL server can be done in Script task. 

So the question arises, why not do everything in Script task why learn so many tasks and transformations. If you are a developer you might like to do everything in a code for your simplicity. But remember, SSIS Tasks are designed for specific purpose and cater to almost all the requirements like logging, trouble shooting and of course the performance. If we write the script for this it wil be time consuming and reinventing the wheel. So make use of the available tasks where ever possible.

If you have a requirement that cannot be fulfilled by SSIS tasks/transforms look for custom components or try building one if it is a common requirement. Else go for a a Script task to get your work done. Remember this simple rule.

The same rule is also applicable to Script Component.