You unlock it once by either buying it from the cartel market, or by buying it from the GTN and binding it to your character. You can then pay some additional cartel coins to make the item or armor set available to all the characters on your account.
Cost to unlock items or armor sets vary from about 60 CC to 600 CC, depending on rarity and original price of the item. For example, a cartel market armor set that normally costs 1440 CC will probably cost around 600 CC to unlock across your entire account. That is cheaper than buying a second set, and now any companions across your entire account can also wear it. Meanwhile something like a mount or less rare cartel pack armor set will be a lot cheaper, ranging from 60 to 480 CC.
The collections system allows you to see everything that is available in all the various cartel packs without going to an external website. It allows you to preview all those items and see what your character would look like with them. It also saves you money if you want the item on multiple characters.