Moov Atom and Network Optimization option

I have a question about the "Network Optimization" option on the Delivery page, and why it isn't the default.

I've just been informed by a developer at our company about the "moov atom" in video files (embedded "metadata" code for playback instructions). I can't believe I've been doing this for 14 years and have never heard of it. The moov atom is located at the end of a file's code, which means when a video is hosted on a webpage and is used in a web banner (for example), the entire video has to load before playback begins. During export, if the "Network Optimization" box is ticked, it will move this snippet to the beginning of the file's code, which allows the video to begin playback sooner before the entire thing loads.

My question is to someone who might know more about this. I would like to know, does ticking this box have any negative effects to the video export? I'm trying to figure out why on Earth you would ever NOT want this box to be ticked, and why it isn't either the default option, or just baked into every export anyway. Am I missing something? I don't want to use the option for everything if there are negative effects, but I work for a marketing company that focuses mostly on web development, so the majority of my work will need this box ticked.

Makes 0 difference for final quality etc.
It’s not on by default as you need to predict its size when you start writing file. Old days with slow internet any useless data in-front was causing longer buffering and delayed playback. Today I don’t think this makes any difference as size of that atom for eg. 1h file is way below 1MB if I’m correct. If you work with files stored in eg. S3 then having atom upfront is very useful. I rather would prefer all masters ( not just streaming ones) had it upfront.
Depending on the design having atom upfront may require re-writing file again ( ffmpeg does it this way), so it’s slower and takes 2x space on the disk for some time. Also with atom at the end you can add extra metadata after file been exported. In other case you may simply have no space to add eg. timecode info. Some tools move atom to the end in such a case. If you were to add extra data upfront then file would get broken as there is offset written in that atom which addresses each “frame” within file. You would need to adjust this info in case of adding extra bits at the start of the file ( this is not that easy I think). If atom is at the end you have no such an issue as you changing data after main vide/audio bits ( so you can write as much as you want).

Thank you for the reply, Andrew. Forgive my ignorance, but in what situations and using what methods would one add additional metadata to the file after it had already been written? I've never had to deal with this, myself.

Metadata is so underestimated. It can be simple thing like missing TC, reel, field order, etc. through comments, keywords up to rich XMP metadata with markers, GPS data etc.