Paintball gear is an investment, you need to protect that investment and make sure it does not get damaged in storage or in transit, so it is always in great shape when you need it most! Cases for your goggles, tank, hopper, marker, lens, regulator for example all help to ensure that your items are not going to scratched, banged up, or just flat out broken while your gear is not being used. There are a variety of manufacturers that produce different style cases, but these all typically have a soft lining that help to avoid scratches, and they usually have additional storage space for related items.
Goggle cases, as the name implies are designed to keep your goggles safe. These are made to be one size fits most but are usually pliable enough to flex if your mask is too large. The majority of the goggle cases that are available will have a pocket on the front to store cleaners, goggle parts, and some are even large enough to fit a spare lens in. These cases can be quite large, but luckly enough, they usually come with a clip that can be easily attached or removed from your gearbag, so it does not take up too much storage space.
Tank cases are usually designed to fit the most common sized tanks and usually fit any from a 13CI up to a 77 / 80CI. Most paintball tank cases will have a spot to place your regulator in the event that you are traveling and need to have it removed before doing so, depending on the case it may also have a pocket to place thread protectors, extra burst disks, or other tools / spares that may be needed. Like the goggle cases, it is common for tank cases to include a clip to attach it to the outside of your gearbag.
Most hopper cases, in addition to holding your hopper will have battery slots for extra batteries, a pouch for rain lids or speed feeds, and will commonly have some form of a molded insert that keeps your hopper in place along with any extras that you store with it. Hopper cases will fit most hoppers, however if it is oddly shaped, overly tall, or overly long it may not fit in one of these cases.