Because it would take about 15 seconds for you to find the answer by yourself by invoking the aforementionned command.

Seriously, here's the first DESCRIPTION paragraph of man sftp:

  sftp is an interactive file transfer program, similar to ftp(1), which performs all operations over an encrypted
  ssh(1) transport.  It may also use many features of ssh, such as public key authentication and compression.
  sftp connects and logs into the specified host, then enters an interactive command mode.