Because noone likes waiting for their Tee.
$ fcat /tmp/largefile | itee | pv -r > /dev/zero
[11.6GiB/s]
$ fcat /tmp/largefile | itee /dev/zero | pv -r > /dev/zero
[14.3GiB/s]
$ fcat /tmp/largefile | tee | pv -r > /dev/zero
[2.90GiB/s]
$ fcat /tmp/largefile | tee /dev/zero | pv -r > /dev/zero
[2.66GiB/s]