Fat is the Thicker version of Normal at the same time heavy String among Kitty String. This has 12 strands of thread but still make a good whip and slack.
Most slacker like this Type of String. They could feel more the Strings moving around for what they want. Many yoyo players uses this also for wide gad yoyo which giving it a more responsive feel, for tight binds (which means your next throw is going to have some intense spin on it).