After discovering that I could create files with the cat command like this:
I was wondering what the "_ACEOF" does.
I tried replacing it with "EOF" and it also works.

So why use _ACEOF instead or any other EOF statement?

There's no reason--it's completely arbitrary.

That said, I imagine the Autoconf developers deliberately chose to use _ACEOF to avoid problems. It's common for compiler developers to use an underscore as a prefix for certain compiler directives. Doing so is an attempt to create unique identifiers without using words/phrases a programmer might like. For instance "_DEBUG" or "__DEBUG" might be used by the compiler because it's extremely unlikely that a programmer would want to use that as a variable or function name. The autoconf developers are likely following that naming convention.

Also, the "AC" likely stands for AutoConf. Another layer to prevent a naming conflict.

If I'm not mistaken, the autoconf package allows the developer to include shell scripts of his/her own as part of the configuration process. Assuming that is the case, you can see why the autoconf developers wanted to avoid a naming conflict. Using "EOF" in way you demonstrate is common, and might easily be used in a script given to autoconf by the developer. There would be a very real risk that the configure script would go bonkers if the autoconf-generated scripts used EOF while the developer-included scripts used EOF as well.

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And why is there sometimes "cat <<\_ACEOF"?

I also currently have a problem in my script:
The cat command seems to add everything in the script coming after the _ACEOF to the created file instead of considering it as part of the script.
I'm trying to create a shellscript that creates a file if it doesn't exist.

edit: Just solved that problem: There was a space after "_ACEOF".
I still don't see why "\_ACEOF" is used. Also for uniqueness?