Isn't ?= needless?

Hello,

I am learning Regex.
I use Regex for a file-searching programme in Linux. (FSearch)

I do not understand, why we need ?= in Regex.
It looks for me like ?= is needless.

Why do I think so?

Here is an example:

abc(?=.*xyz)

But isn't this exactly the same like:

abc.*xyz

So actually my question is:
Can you give me an example, where ?= is really needed, because I can not get the same result without ?= ?

Would appreciate some answers.

Thank you.