What is use of command: `command`?

command is a bash builtin as we can see:

seth@host:~$ type command
command is a shell builtin

So we know command is provided by our shell, bash. Digging into man bash we can see what its use is:

(from man bash):

command [-pVv] command [arg ...]
              Run  command  with  args  suppressing  the normal shell function
              lookup. Only builtin commands or commands found in the PATH  are
              executed.   If the -p option is given, the search for command is
              performed using a default value for PATH that is  guaranteed  to
              find  all  of  the  standard  utilities.  If either the -V or -v
              option is supplied, a description of command is printed.  The -v
              option  causes a single word indicating the command or file name
              used to invoke command to be displayed; the -V option produces a
              more  verbose  description.  If the -V or -v option is supplied,
              the exit status is 0 if command was found, and  1  if  not.   If
              neither  option  is  supplied  and  an error occurred or command
              cannot be found, the exit status is 127.   Otherwise,  the  exit
              status of the command builtin is the exit status of command.  

Essentially you would use command to bypass "normal function lookup". For example, say you had a function in your .bashrc:

function say_hello() {
   echo 'Hello!'
}

Normally, when you run say_hello in your terminal bash would find the function named say_hello in your .bashrc before it found, say, an application named say_hello. Using:

command say_hello  

makes bash bypass its normal function lookup and go straight to either builtins or your $PATH. Note that this function lookup also include aliases. Using command will bypass both functions and aliases.

If the -p option is provided bash bypasses your custom $PATH and uses its own default.

The -v or -V flags bash prints a description (short for -v, long for -V) of the command.

Note: As souravc pointed out in the comments an easier method for finding information about shell builtins can be found here: How to make `man` work for shell builtin commands and keywords?

This is built-in command of the Bash shell.

The only advantage I see with this built-in is summarized in the following sentence of the help text:

Can be used to invoke commands on disk when a function with the same name exists.

So if you want to execute a program (a binary file saved somewhere on your disk), and an internal shell function of the same name exists, then you can invoke your program using this built-in.

And yes, command -v will give the same kind of result as type.

I've found it also under the Dash shell.

It has two different uses:

One use is to ignore aliases and functions, and run the executable file found in PATH, even when an alias or a function with the same name exists.

As example, I'll use an alias for ls that appends a / to directory names:

$ alias ls='ls --classify'
$ ls -d .
./
$ command ls -d .
.

In an interactive shell, it may be more convenient to use a backslash before the command name as alternative, shorter syntax:

$ \ls -d .
.

The other use is to find the command that will be run when the commands name isn't used by using the option -v. It seems to be the most portable/POSIX variant of which.

$ command -v ls
alias ls='ls --classify'
$ command -v sed
/bin/sed