Why use Spark with CI?

Spark is long dead. Check the date of the tutorials that mention it.
If you want to handle dependencies, look at Composer instead.

Thank you. Can you tell me? What are situations I should need Composer? For example, Ecommerce, App, or for all websites?

Think of composer as a spark for all of php. You have access to a massive amount of libraries that aren't directly created for codeigniter, but for multiple platforms on php. You can uses libraries from other frameworks like Symphony, Cake, or Laravel in codeigniter or stand alone libraries like the Facebook SDK.

Codeigniter is simply one of the tools you need to learn to be a successful developer. Always add more tools to your coding arsenal!