First and foremost, it looks incredible. Colour matching your cables to your build can be the finishing touch that really makes your build “pop.” A custom build can be visually unappealing with huge, clunky and hard to manoeuvre cables. Sleeved cables offer a level of professionalism to your build, and make people looking at it go “whoa!”
Secondly, it increases airflow. Those big OEM cables that come sprawling from your PSU in a huge mess actually restrict air movement in your case. As you know, heat is one of the huge major killers when it comes to electronic components; you’re going to want them to run as cool as possible. Sleeved cables allow air to pass over, under, around or through them with significantly increased proficiency compared to standard cables.