Why the file /bin/sh point to dash shell but my default shell is bash?

Question

When I type the command:

ls -la /bin/sh

I have this output:

lrwxrwxrwx 1 root root 4 Feb 17 12:26 /bin/sh -> dash

So the file /bin/sh is a symlink pointing to the dash shell program.

But when I check what is the current shell program I'm using by typing this:

echo $SHELL

I have this :

/bin/bash

So I'm using bash but why the file /bin/sh is pointing to the dash shell program? What is the used of /bin/sh?

My current Linux kernel version is 5.15.0-1025-raspi