From https://wiki.archlinux.org/index.php/SFTP_chroot, emphasis mine:
Write access to chroot dir
...if a user is able to write to the chroot directory then it is possible for them to escalate their privileges to root and escape the chroot. One way around this is to give the user two home directories - one "real" home they can write to, and one SFTP home that is locked down to keep sshd happy and your system secure. By using mount --bind you can make the real home directory appear as a subdirectory inside the SFTP home directory, allowing them full access to their real home directory.
This can also be used to achieve other goals. For example, a user's home directory can be locked down per the sshd chroot rules, and bind mounts used to provide users access to other directories:
# mkdir /home/user/web
# mount --bind /srv/web/example.com /home/user/web
Optional add an entry to /etc/fstab:
# echo '/srv/web/example.com/ /home/user/web none bind' >> /etc/fstab
Now the user can log in with SFTP, they are chrooted to /home/user, but they see a folder called "web" they can access to manipulate files on a web site (assuming they have correct permissions in /srv/web/example.com.