These are not "freebies" per se. These are samples mailed by request from a retail packaging company. They send samples to businesses in the hopes you will purchase product from them. These are usually business to business offers.
Some of the freebies you will find here are promos like the GM t-shirts, there will be a direct tie-in to the company and a product they are launching. In the case of the GM t-shirt it was the new ethanol hybryd vehicle. Promos are usually while supplies last.
Some freebies, like the pet and beauty supplies are to incent you to try the product , like it and buy a full sized version. There are charitable freebies tied into a request for donations and governmental freebies tied in to educational purposes, etc...
If you come across a free offer that seems too good to be true, it probably is. A free offer that is on a stand alone page with no other website pages you can access explaining the offer (or a "contact us" option) are usually a ruse to get you to participate in third party offers. You will not get your bag of chips or ipod mini until you buy something .. That's not really free, is it.
There are a lot of fun and valuable offers here in this forum. Take your time when you review each offer and use your judgement.
Happy Freebie Hunting!