Are the DNS servers really necessary, especially when I watch the videos such as the advertisements?

Have a look at https://nextdns.io/

DNS is VITAL. Without them, you can't surf the web.  They are the phone book. You ask for answers.yahoo.com.  IP doesn't know how to go there.  It needs and address. So it asks the DNS (domain name server so saying DNS Sever is pointless)  for the address of answers etc.  Your computer gets an answer like 76.13.32.153.  You can put that in the URL bar and it will go there.

As for blocking advertisement, sure, DNS can do that.  Mostly by lying about the address of ad servers. Few DNS servers do that. One that can is OpenDNS.  It can be configured to block most bad stuff.  It is near vital if you have kids.

i think you are either confused, or have been lied to.

a DNS server does not block ads. it is the phone book of web server addresses, basically.

it converts text addresses into the server's IP Address (like a phone number for computers), so that YOU don't have to remember 77.153.128.46, instead of YouTube.com

kind of important, don't you think?

if you want to block ads, use an Adblocker, like AdBlock Plus, or use a custom HOSTS file, like this:

https://winhelp2002.mvps.org/hosts.htm