/etc/wpa_supplicant.conf present but ignored. Generated wpa_supplicant.conf is used instead

I have this error while setup networking on NixOS livecd

Create wpa_supplicant.conf with one liner command then start the wpa_supplicant.

wpa_passphrase SSID PASSWORD >/etc/wpa_supplicant.conf
systemctl start wpa_supplicant

The error from systemctl status wpa_supplicant

/etc/wpa_supplicant.conf present but ignored. Generated /nix/store/xxx-wpa_supplicant.conf is used instead.

Why the behavior is like this? I do like setup the networking with one liner than the one from [nix manual](https://wpa_passphrase SSID PASSWORD >/etc/wpa_supplicant.conf).