2 IP addresses, no router

iPhone 6s Ios 11.3

(fw) 4.56 and I-boot 4076.50.126

i have one each of default gateway IP, default gateway IPv6, and external IP however I have 2 DNS server IP’s. I googled what they were and saw a little drawing of the Path they take to one another and how they get there but I don’t understand having two server addresses. whats the second one do? I don’t own a router or computer, keep my WiFi toggle set to off at all times, And I’ve only Plugged my phone into a wall or carport charger. I also don’t own a Tv, sound enhancing speakers, smartwatches, fit bits, iPods, MP3 players, or anything else capable of plugging itself into the phone. I’m not concerned, just want to learn more about it and what it’s capable of. Thank you