Sam, sorted now - see thread. as always very grateful for the A1 service you guys at OSMC provide. Brilliant
There is not really a good short answer to this. When set to your own router it usually is the same as manually putting in the dns from your isp. Some routers let you set custom ones, and can provide extra functions but this is entirely dependant on your router. The reason why someone may want to use google (184.108.40.206) is to get faster speeds or remove redirects or blocks that may be issues with the dns provider from their isp (it also may be slower fyi). Another common one people use is cloudflare (220.127.116.11) because it is fast and they claim they make zero logs for privacy reasons.
Glad to hear all is working now.