I expect that using the iCUE software is better, but not required.
When you install that system, the instructions are to connect a cable from the pump to the CPU_FAN header, and the cables to the RAD FANS do NOT go to the mobo. Instead they plug into output connectors on a cable from the pump. When you then run the iCUE software, it knows that the PUMP speed will be read from the CPU_FAN header where the cable is connected, and the FAN speeds will be read from the pump unit itself - there is also a cable from the pump to a mobo USB2 header for communication with iCUE. iCUE controls the FAN speeds that way as well as monitoring them for failure. iCUE also monitors the PUMP for failure (VERY important) from its speed signal on the CPU_FAN header. It also allows you some options for setting pump speeds for fine-tuning its controls, a feature many AIO systems do not have. But most importantly, any cooling control system has a few "tuning parameters" for system response times and amplifier gain, etc. that you never see, and these are pre-set by Corsair exactly suited to the way the H150i system operates. ALL cooling of the CPU is controlled by iCUE and you can customize that in that software. The BIOS display of "CPU Fan Speed" will really be the PUMP, and there will be NO ability in BIOS to see and show the rad fans - that is done in iCUE.
If you do NOT install as instructed, then you NEED to change how connections are made so the mobo normal control system can operate. First, the cable connection from the PUMP to the mobo should go to your mobo's PUMP header. That will operate the pump at full speed all the time, monitor and display its speed for you, and check it for failure. Then the RAD FANS need to be connected, using a simple SPLITTER, to the CPU_FAN header. That will control their speeds according to the CPU chip's cooling needs and monitor one of them for failure while displaying its speed. You will NOT need the cable to connect to a mobo USB2 header if you NEVER want to run iCUE. Then all control of CPU cooling is done by your mobo and you can customize that in BIOS Setup if you wish. This does mean, though, that the "tuning parameters" it uses are those suited to a common fan and heatsink cooler, rather than the AIO system. Not a big deal, but it is a difference.