One of the reasons I regularly (maybe once per month) test for nitrate, is to let me know if my water change schedule is adequate.
If I get a few 1-2" oscars, and put them in a 75 gal tank, a once per month water change may be able to keep nitrate at 5 ppm or less in the beginning.
But as these fish grow, two weeks in-between water changes may not be enough, I may find nitrate levels have crept up to 10ppm or higher (water changes are the only reliable way to keep nitrate at safe levels, filtration does not).
This tells me my water change schedule and amount may not be up to par for that size tank.
The other way I might find out, weeks later, is my oscars may be showing signs of HITH disease.
The test would have let me know much sooner.
And later when those same oscars are 5" or more, when nitrates may be hitting 20ppm or higher, to keep nitrates from creating chronic disease, testing might "suggest" a weekly or bi-weekly 50% water change schedule, or "suggest that 75 gal tank does not have sufficient water volume for those larger fish.
The disease section has any number of posts where people state "my oscars, or other fish were fine for 2 years, but now they are scarred up, or worse".
A regular test regime, and analyzing what the information those tests tell, may have prevented may of these disease type posts.
Same goes for pH, it lets us know when acidification from excess nutrients in the water is occurring, and that water changes are needed.
Experienced aquarists may easily recognize some of these things without testing, but aquarists with only a few years of fish keeping under their belts may need help, and that's what water testing provides.