People bind their breasts for all kinds of reasons. For some people, it's a part of helping themselves to look the way they feel they most want to look; this might be an aesthetic choice, or a part of someone's gender expression. For others, it's like a costume - a way of being someone else for a while, or of taking on a certain role. Genderqueer and non-binary folks might bind in some situations but not others, or on days when they feel their gender expression will benefit from it. Binding isn't usually overtly sexual, though many people who bind find it more comfortable to remain bound during a sexual encounter.