The main reasons why companies give away free stuff include: Promotional giveaways - this can help increase sales and also increase there customer database by giveaway prizes.
Feedback on new products - If they have a new product launch they may give away samples to find out what people actually think of it.
Build a customer database - they can get new customers info and may promote to them in there newsletter. Build a better reputation - Big brands often giveaway samples in exchange with reviews to find out what people actually think of them.
Infographic by: justfreebies.com