We can start with the fact that German nouns (e.g. job titles) have a grammatical gender. Due to that offers included "m/f" in the past. Since this[0] ruling companies prefer to be safe.
So in short: legal safety and following the German law [1].
That's why German based companies include "all genders" or "m/f/d" in their job listings.
[0]. https://www.bundesverfassungsgericht.de/SharedDocs/Pressemit...
[1].https://de.wikipedia.org/wiki/Allgemeines_Gleichbehandlungsg...
> Software Engineer (all genders)
What is the significance of specifying genders? Why would male or female make a difference?