There are at least twol reasons to read reviews of equipment you've already bought.
First, if you are like most on this forum, audio is one of your hobbies. You enjoy reading on the subject, whether it involves stuff you already own, items you're thinking about buying, or esoteric gear that you'll never have a chance to own. All of it is just interesting to you. The same is true of car nuts, knitters, sports enthusiasts and any other hobby out there.
The second reason for reading a review for something you already own is to see if the reviewer's opinion lines up with yours. It can be very useful to know if a particular writer's viewpoint matches yours or if you think they are full of it. That can be very helpful when they review something you don't own, but are considering. If they are in your camp, or out of it, that'll give you a good indication of how seriously you should take their comments.