Every download site works the same way at the start. There's no upload button, no way to drag a video in, just an empty box waiting for a web address. Anyone who wants to baixar video youtube por linkhas to go find that address first, copy it, and hand it over before anything else happens.
The Link Is the Only Thing the Tool Ever Gets
A download tool never receives the video itself, not even for a second. It receives a string of text pointing at where the video lives, and everything that follows depends entirely on that pointer being accurate.
This is different from how most file tools work. A photo editor opens an actual image file sitting on a device. A video downloader opens nothing. It reads an address, sends a request to YouTube's servers using that address, and only then does any video data start moving at all.
Compare It to Asking Someone for Directions
Handing over a link is closer to giving someone directions to a house than handing them a key. The directions don't contain the house. They just describe exactly where to find it, and someone still has to make the trip.
A download youtube video by link tool makes that trip automatically the instant a link lands in the box. It follows the address, arrives at the video, and starts requesting the pieces needed to rebuild it as a file. Nothing about the link itself carries any video data. It's pure location, nothing more.
What Goes Wrong When the Link Is Slightly Off
Paste a link to a channel page instead of a specific video, and the tool has nowhere to go. Paste a link with extra characters tacked onto the end, common when copying from a playlist, and the request can land on the wrong video entirely or fail outright.
This is why so many of these tools show an error immediately rather than a slow failure. The request either resolves to something that exists or it doesn't, and there's no in-between state where a vague link produces half a video.
The Absence of Any Other Method
No widely used tool asks for a video's title and searches for it independently, and none ask for a channel name and a rough description. The link remains the only accepted input because it's the only piece of information precise enough to identify one specific video out of billions.
That precision is exactly the point. A search-based system would guess. A link-based system like baixar video youtube por link doesn't have to.
One Small Piece of Text Doing All the Work
Everything downstream, the file format chosen, the resolution offered, the speed of the process, depends on that single piece of text being correct from the first paste. Get the link right and the rest happens automatically. Get it wrong and there's nothing for the tool to build from.
That's the entire reason the box asks for a link and nothing else. It isn't a design preference. It's the only input that actually tells the tool where to look.