Note: Once you have downloaded an icon collection you can use other applications or an icon editor to make them glossy or different.
good icon will be small and easily understood. For example you can use various icons to represent a shopping cart for a check out page on a website where the buyer is actually choosing items for their shopping cart to purchase at a later time. You can order a collection of icons to make that website more distinct with a slightly different look than other websites selling that same product.
So purchasing an icon collection can give your business or online retail store a different look. The website where you just downloaded that icon collection is also a business and it wants your repeat business.
Tip: You can also purchase different security icons or directional icons depending on what you are in the business of selling or providing.