Something I never really understood is how and why some rod manufacturers print line size ratings on their rods. What's the reasoning behind this? For instance I have a micro guide Denali that's rated 3/8 - 1 oz lure and 12-18 pound line. I throw 20 pound mono on it and it performs great. Is this just a basic recommendation for ideal performance or does it have something go do with guide size?

Has more to do with the strength of the rod.

You can go higher with no problems. Going lower can cause a lot of broken line.

I always thought it was for things like warranty. If you used too strong of line the rod might break and they could say you exceeded the ratings.

There was a post earlier this year about this and the relationship of reels being hard to tune backlash out. Since then I have more closely followed the line info and lure weight. My backlashes have decreased tremendously! And, casting distance has improved.

Is was watching an Aaron Martins video the other day where he said that he sets the hook the same with different line weights and the rod is set up so it never breaks off. Hard to explain, but it was the last two minutes of this video.
https://www.youtube.com/watch?v=x-WiQ-C1D70