It requires the molex connector to power the 3 fans that it can connect to. If it were a two-way splitter, it could power two low-power fans, but with three, you need a molex connector to give the fans some more juice.
Actually, that splitter makes sense. Here's how the 4-pin connector on the mobo is set up, with "standard" wire colors to the fan:
Pin #1 is Ground (Black); #2 is + 12 VDC( Yellow); #3 is speed pulse signal traveling BACK from motor to mobo (Green); #4 is the PWM control signal (Blue).
The mobo connector has a tongue sticking up by Pins 1-3, but NOT at Pin 4. The connector on the wire ends has a slot to match the tongue, forcing it to fit on only one way. (By the way, 3-pin fans and connectors use the same first 3 pins - on them Pin #4 is just missing. For speed control they depend on the mobo reducing the +12 VDC voltage.)
The way the 4-pin fan works is that it gets a +12 VDC and a Ground power supply, and a PWM pulse signal. That PWM signal is used by a tiny control circuit INSIDE the fan case to regulate when current from the power lines is actually flowing through the fan motor. As it turns, the motor also generates an output signal of two pulses per revolution that is sent back out to the mobo as the speed signal.
The dilemma for people who want to connect several fans to a mobo pinout is the heavy start-up current - specs allow up to 2.2A per fan, and many mobo fan outputs cannot supply that much to more than 2 fans in parallel. But that is for the POWER required, supplied only by the Yellow and Black lines. It is NOT a problem for the PWM control signal.
What this splitter does is take the Ground and +12 VDC supplies from a Molex 4-pin PSU output instead, which CAN supply many amps at start-up time - 4 fans COULD pull up to 8.8 A for a second or two. But the PWM signal is supplied to ALL of the fans. So, each fan has a good reliable power supply and a control signal it shares with the others. And they all will run at pretty much the same speed.
Note also that only ONE fan connector has the Green speed signal line attached (the one marked "CPU Fan"). ONLY this fan's speed pulse output signal will go to the mobo for measurement. The other fans' speed pulses are NOT connected to anything, and that is exactly the way it should be.
The limiting factor here is that ALL fans will run at the same speed. What speed? That depends on which mobo pinout you plug into for the leads labeled "connect to motherboard". If you plug that into the mobo CPU_FAN pinout it will all be controlled by the CPU temperature and cooling needs. And most certainly, if you are running your CPU cooling fan off this splitter, that is what you should do. Also ensure the CPU cooling fan is plugged into the output connect so labeled.
IF you are powering and controlling the CPU fan on its own from the mobo's CPU_FAN pinout, you could use this splitter to power up to three case fans by plugging the "connect to motherboard" line into a SYS_FAN pinout. In that case, make sure it is one that actually DOES control case fans by measured temperature from a sensor on the mobo. (I have seen mobos that actually CONTROL a case fan speed on only ONE of their two SYS_FAN pinouts, and run the other always full speed.) If you are doing this, just plug your third fan into the splitter's lines labeled for CPU, since you already have the CPU fan powered and controlled separately.