Thanks all the people for their help.

In that even he even shows why he did it directly afterwards: in order to do some construction.

And here are yet 3 nother reason for outlining the stroke: this video is about logos and with logos you have to do it:

- because logos often need to be cut with a plotter

- because logos are often opened in other vector applications and you never know what they will do with your carefully chopsen stroke width

- because you don't want the people from marketing change your stroke to dashed line just because they think it's fancy

Maha,

There are (sometimes crucial) reasons to either keep a stroke or to change it to a fill.

Always keep a backup of artwork that you are going to change irreversibly, such as outlining stroke/expanding to get fill instead, so you can go back. It is also easier to keep the stroke until you are sure about the shape of the (spine of the) path.

In connexion with logos, there are (other) specific reasons to end up by outlining strokes, such as to ensure the final appearance and the usability across whichever applications/formats/uses may prove relevant.

Bobby mentions some applications/uses where the usability reason applies here,

https://community.adobe.com/t5/illustrator-discussions/outline-vector-cannot-be-read-from-illustrator-2023/m-p/13384052#M345650

Sometimes I've converted strokes to paths in order to get to parallel lines on a complex path.

One reason is to manipulate the paths once the outlines are converted to fills.

Once you convert your stroke into a solid fill, it is “immune” to scale effects. It will always keep its shape as you intended it to be. As long as you have a path that is only a stroke you could change the stroke width while scaling. This setting can be found in the Transform Panel.

Also: If you want to create a font out of a shape it has to be outlines (no strokes) only.