The difference between the two is dependent on what you want
The insert object only will put one object in the scene for you. Uses of this are individual objects that you want to manually place, like houses, very unique trees, etc.
The population will use a population to allow you to create a ton of trees within an area very simply, with varations of rotations, sizes, etc. This is useful for grass fields and forests.
Also, when you first import either of them, they automatically load onto 0,0,0, which most often is not where you are looking at. You need to either move the population over (easy way is to right click in 3d preview and copy coordinates, then use the paste coordinates into the population location), or if its just 1 object, you can do the same thing and just copy it to that one location (you might need to adjust the y location for height on individual objects), but populations automatically will sit on terrain if done correctly.
*hah, while modifying got beaten a bit below me
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