Wien Netze and WIen Energie (Vienna)

Can someone please explain to me the difference between Wien Netze and Wien Energie. Why do I need to send info to Wien Netze as well as Wien Energie?

I moved recently and I organized that I pay my monthly fees for electricity and gas to Wien Energie, which is an automatic withdrawal. However, I received in the mail a "Netznutzungsvertrag" from Wiener Netze and an "Energieliefervertrag" from Wien Energie. I am suppose to send back these letters signed.

I don't understand what for? The answer is probably so simple but I asked Wien Energie about the letter from Wiener Netze and they just said it was a different company and I needed to talk to Wiener Netze directly. Hmm...what is going on?

I would appreciate any info.

Thanks,
Nina