How Weblinks Work
The main menu to the right uses the Wayfinder snippet. The same menu below uses the older DropMenu snippet. Mouse over the Weblink Test item, and see the difference in your browser's status bar or tooltip that displays the URL for this link. The main menu is linked directly to the desired URL, while the older DropMenu menu is linked to the weblink document, then is redirected to the desired URL.
[[DropMenu? &startDoc=`0`&levelLimit=`1`]]
At its most basic, a weblink is a document whose content is a URL. It is assigned a type of "reference" in the database. When the MODx parser sees that a document is a "reference", it immediately uses the content as the URL for a redirect, without any further parsing. So these documents have no templates, no TVs, nothing. Their only purpose is to allow the menu snippets to add the link to your menus.
I get clever
It was an annoyance when wanting to link to a MODx document outside of the path specified in the menu snippet (the startDoc or startId value), since you had to know the full URL of the document you wanted to link to. So some time ago, I hacked the parser to check if the content was a number. If it was, then the parser would use the makeUrl($id) function to create a proper URL, then send the redirect. And just for fun, I "borrowed" the javascript code that allows you to click on the document in the Document Tree to set the parent of a document, and added it to allow clicking on the Document Tree to set the document Id you want to redirect to. You can use the guest manager login to see what I mean.
Menus get smarter
Then the authors of the Wayfinder snippet got tired of this redirect business, which meant that the link displayed for the page in the menu was the address of the weblink document, not where you wanted to go! For example, if you had a weblink to google.com, the link displayed would be to the weblink document, which could be "index.php?id=543", or "go-to-google.html", instead of google.com. So they modified Wayfinder to check if the document it was addding to the menu was a "reference", and if it was, to use the document's content to make the link. This had two advantages, it not only caused the proper URL to be displayed (in tooltips or the status bar of the browser), it also eliminated the redirect, and an extra call to the parser with an accompanying extra query to the database.
Oops...
But this totally killed my clever modification to allow the use of a document ID for an internal link! Now, since Wayfinder uses the content of the document, you would get just the document's ID as the URL. Like, http://www.mysite.com/456. Holy 404, Batman!
After fiddling around with converting the document ID when saving the weblink (and what if you change its alias later?) it was finally decided that Wayfinder would do a check on "reference" type documents, and if its content is a number, run it through $modx->makeUrl($id) before using it to generate the link.
The parser itself still checks for IDs and converts them to URLs on parsing, so older snippets and any others that are OK with the redirect method aren't bothered by it. But for any snippet that wants to skip that step and use the Weblink's content directly, it must be passed through $modx->makeUrl($id) if it's for an internal link and its content consists of the desired document's ID.