Paintball chest protectors are commonly worn under a jersey and help to dramatically reduce the impact of a paintball, helping to avoid welts and a good amount of the pain that comes from the impact of a paintball. There are a variety of styles for chest protectors, some cover the chest, shoulders, and back, others may cover only the sides, and some have built in elbow pads. Overall chest protectors are a “personal preference” item, some people prefer to wear them, others do not. Since you are adding additional layers these definitely can retain heat, so depending on the climate you are playing in, you may want to consider that. In most tournament series you are allowed to wear 1 layer under a paintball jersey, and a chest protector would count as that one layer, so they are commonly legal to be used, but of course you may want to check with the league that you are playing in.