Many of the components out there you can either use the colon, or the DP, but not both. I am baffled why the manufacturers would do this.
Here's my guess, it's pure speculation.
If you look at the pin layout for a typical single 7-segment LED you will find out that it has 10 pins. Since they use two of them for the common connection and seven more for the segments that leaves only one free pin for either a decimal point or a colon.
Why do they need two common connections and why don't they replace one of them with the missing colon (or decimal point) you ask? That's because the pinout is arranged so that the device can be plugged in upside-down and the seven segments will still function as intended due to the symmetry in the pin layout.
Why do you want this symmetry? Because if your display is equipped with a decimal point you can use two of the devices, side by side, with one of them upside down and the two decimal points will now look somewhat like a colon!