One of the basic skills we hope our beginning freshwater hobbyists will come out of the "new freshwater tank" section with is a good understanding that nitrate(NO3) typically really needs to get removed via a good weekly water change habit and that that is an important responsibility for the beginner. Although it is true that heavily planted tanks may have significant portions of nitrates removed by the plants, by far the typical thing going on in many of our forum discussions are relatively new hobbyists (or for that matter older hobbyists who just haven't paid attention that much) still needing to fully understand the various benefits of regular water changes.
In fact, another interesting bit about nitrates I've always thought is that the particular substance, and it's test, really serves as a "canary in the coal mine" sort of signal. Tank water that is not changed regularly will sit around and evaporate. When that happens, the minerals, nitrates and actually hundreds of other things just stay in the tank. For the most part only the pure H20 evaporates out. One way to get a guage on the level of "stuff" that has stayed in the tank is to be watching your NO3 over time. Everyone's baseline for this will be different, mostly due to the differences in source water. But the important thing is that if at some point you see nitrates make an unusual climb (and often if one is honest with one's self, it will correspond with those times when you've vacationed for multiple weekends or whatever, lol) it can be a tip-off that water changes (with good technique) are in order to return your water to a state that is fresher and that more closely matches the chemistry of your source water.
(sorry, don't know why I feel so wordy this evening!)
~~waterdrop~~