2 does seem redundant.
I guess Security Centre is there as a Overview of security on PC, its not just AV & FIrewall, it also shows Smartscreen settings, whether you have secure boot on, and general health of PC + family settings. You can refresh PC from this menu (similar to a reset)
Defender just has the settings for AV & firewall
It makes sense to have them as separate apps as you can have Defender act as Firewall while you use another AV
or disable defender completely and still use Security Centre for its other functions
also means Defender can be updated without updating the security centre as well. Probably less complicated for Microsoft.