what is the purpose of a checkpoint?

[Ali Khalfan <ali.khalfan@gmail.com>]

I tried to backup a directory using an archive called sample  using
"tarsnap -c -f sample /home/user/dir1".

Later I send a USR2 signal and then break the backup using ^C.  When I
run the same tarsnap command again, I get an error saying it already
exists as sample.part.

What should I do in this case?  how do i continue the backup on the
.part file since I've been running the backup for days, I'm assuming
there is something on the server.