Single-line comments have the benefit that it is always clear to the reader which lines are comments. Whereas if you use multi-line comments to comment a large section of code it is no longer clear when reading it whether it is indeed "inside" a comment.
And finally having both types of comments available is useful because you can multi-line-comment a section which already contains // -comments, whereas you can not multi-line comment a section which already contains multi-line comments.
// => ////
//////////////////////////////////////////////////////////////
// just like those devs that like to decorate their code with
//////////////////////////////////////////////////////////////
/*
* type of stuff
*/
Edit: almost forgot my shell friends
########################
## works too
########################
But what about the below, does it work in your language? I assume it may work in some languages but not all.
/*
let a = 1;
/* a is one */
let b = 2;
*/