Better modems usually come with more settings, better support, better safeguards when it comes to power related issues (cheap modems tend to do worse when lightening strikes in proximity to your house) and use higher quality parts in their construction meaning the modem will last longer without causing problems.
I don't know how it is in your place exactly but for example the modems we get here from the ISPs are some very cheap modems that basically need to be replaced every two-three years because they just break or create errors making multiple restarts or resets necessary each week.
A good modem can last 10 years and beyond.
Furthermore if it has build in WiFi the WiFi is usually stronger and of better quality than in a cheap modem