when you select the sequence settings to match your source material it makes the editing software happy because it is dealing with the source material as if it " knows " what it is, how to handle it.. the specs etc.. so when you add transitions and cuts and all that stuff the editor does a really good job.
After you're done editing there may be any number of "platforms" or " pipelines" or "products" you want to make. For example, maybe you want to broadcast it on TV. Maybe you want to save it so you can go to some other "step" in your process ( color grading or SFX ) Maybe you want to webcast it... All of those things you want to actually DO with your edited footage are best served by different export settings.
When you are on your bike on level ground at a constant speed you'd be in a certain gear and exerting a certain constant pressure with legs to maintain your desired speed. That's when you and bike are happy. You are the source material, the bike is the editor...both working well for the purpose...
Then you encounter a hill or something... and you change gears and pressure etc to accomadate the change. That's the export... you export for your purpose...
stupid analogy maybe but sorta might make some sense ?
Actually, it was a good explanation. It makes sense.
I might be able to come up with another analogy that involves really SCARY SNAKES...if you want to hear it ( and if I haven't had too many beers to think of a good one )...