@Thub programming is different from using. What looks logical for programmer not always convenient for the end user (related threads:
https://obsproject.com/forum/threads/obs-duplicating-issues.94415/ and
https://obsproject.com/forum/threads/source-paste-reference-clarification.96165/ and some else).
Detailed explanation
OBS has "Sources" - let's say invisible "objects" that can generate audio or video signals. OBS has "items" - each "item" is a reference of the "Source". "Item" holds visible texture from the source, it can be resized, muted etc. This "items" - is what you see in the "Sources" pane of OBS. When you adding the very first source to the Sources pane you actually doing two actions simultaneously: initializing new "Source" (that itself starts to generate audio or video somewhere in the memory of your PC) and adding visible representation of this source (now named "item") to the Sources list pane (and seeing it in the Preview).
OBS is real-time tool, so it is better to keep number of the "Sources" (objects that generates audio and video) as low as possible. Many of these sources are always running in the background (remember, you only see a representation of the sources - the "items" that reside in the Sources pane) - this continuous running makes it possible to seamlessly transition between scenes. If you making full copy of the source then ALL properties of the source can be changed. It means, that a new object has been created in the memory of your PC and this object now generates a second thread of audio or video data (double the load on your PC!).
When you making reference copy of the source then only the texture is copied, you see it as a new "item". Thus, when you do "Add existing" source or "Paste reference" source in OBS, you are actually adding a new texture (representation) of the "Source".
The "Filter" is a special source that connected in the chain to some original source (not to the "item", because item is only a texture with the size, visibility and mute properties).
The "Group" is a special source that may contain only "items"... and, of course, some "Filters" can be connected to it in the chain same as to any other regular sources.
I hope, you now understand OBS a little better. I may be wrong about some details, but the general explanation should be close to the original logic of the program. And one more thing, by clicking on the "Properties" of an "item" you open the properties of the parent source (the "Source", the "Filter", the "Group" etc).
Short explanation
Some sources cannot be fully copied in a simple way. Some sources (Video Capture Device) are useless as full copies, because one source running = device already in use.
