What's the use of buying licensed software?

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I don't get it. What's the use of buying licensed software if there are sites who offer it for free? Even the software developers offer trial version, unregistered versions of their products. The flaw is many sites give cheats, cracks, registration keys, serial numbers which activate the product as full version, withut expiration! Are their codes crackable, vulnerable to duplication? Can't they not program a software which run only in one computer? Or why not just program all softwares for free? Huh?