Option a) Because I am a generous soul, who believes in charity and goodwill to all men OR
Option b) Because it is in my commercial interests to persuade product manufacturers that it is in their commercial interests to give me product, which I then distribute for free in order to generate reviews for the manufacturer, and drive traffic to my website. Incidentally, this is also in the commercial interests of those who get the freebie (obviously) AND those who read the reviews, thereby gaining free information from a source which they trust. A rare case of win-win-win.
Its one of the above. Damned if I can work out which.