Because a SoC with limited CPU-core resources can't do everything in software, the chip contains many system components (hence System-on-Chip or System-on-a-Chip) that handle things the CPU cores then no longer need to do.

Think protocol handling or memory; instead of spending many clock cycles on handling the USB bus, you can leave that to the USB controller and only deal with what is actually relevant to your USB device. Same with the VOP (Video Output Processor) block, instead of spending many clock cycles on putting the right bits in the frame buffer you tell the VOP that you'd like the background to be orange and then only spend time setting the right bits for black text (for example). So instead of having to deal with many millions of bits, you only have to deal with less than 1% of them because every bit you don't set becomes orange. For other things like I2C, I2S, DMA, networking, cryptography, SD-IO, GPIO, PWM etc. the same applies. Instead of constantly spending time setting the right bit at the right time many times each second, you just tell a dedicated block on the SoC to do a thing in a certain pattern and it will do it for you, consuming on CPU core resources.

This also allows slow CPU cores that wouldn't be able to decode video in real time to offload the entire decoding to a video decoder block, and then tell the GPU part of the SoC that you're drawing a green rectangle somewhere and that's where it has to put the decoded video frames. Why would one do all of this? Because it's cheap and power-efficient, and that's how you make a big pile of money.

I have no idea how accurate or up-to-date this PDF is, but it should at least give you some idea as to what a SoC can do without bogging down the CPU cores: https://dl.radxa.com/rockpis/docs/hw/datasheets/Rockchip%20R... Check chapter 9 for example, all of those boxes are things you don't have to spend CPU cycles on. If you did use the CPU, it would be super slow.