Attachments allow you to personalize and customize your weapons, modifying them to best suit your tastes, and to perform more effectively in a variety of combat situations.
All attachments provide some form of benefit for your weapon, either improving its performance outright, or making it easier for you to personally handle.
Choosing attachments for your weapon comes at the cost of other equipment and perks for your loadout, so put some thought into what you take. There’s nothing wrong with taking a heavily customized weapon into battle, but giving up perk muscle or equipment suitable for a specific objective mode is a real trade-off.