If you-have a Radeon card with a Displayport and you wish to use the Radeon triple screen "eye-infinity" option, order do not-have a monitor with a DisplayPort input (They Are still uncommon), Then You need one of These. Crucially, this device is "ON" not a passive pass-thru, so the Radeon card is happy.
We use this year with HD7770 and HAD already connected the HDMI and DVI outputs of the Radeon card to monitor 1 and 2, for the 3rd THUS monitor this adaptation we needed. Works a treat.