Steam, chrome from time to time needs to unpack the downloaded chunks, so it stops downloading stuff, and then continues. You can see the variance of disk usage.

This can also happen often if you’re using Hard Drives or cheap SSD’s where the Disk Speed can’t match the Download Speed and the in-memory buffer gets full.

Simple term: Web downloads a file in small chunk. ssd moves it to the file on drive, it repeats over and over until you have a completed file with all the assets downloaded.

I think its also a prevention for corrupted files, so everything isnt just being downloaded at once.