These can be one of the most powerful tools an author has at their disposal. For most self-published authors, there aren't a lot of good ways to get your books into the hands of readers. Not only are Goodreads readers willing to take copies of your book, they are usually more invested to read it than other readers as well.
Basically, when people signup to enter your giveaway on Goodreads, one of the offered conditions is to add the book to their Want to Read list. They can, of course, elect not to - and many people do - but Goodreads will try to invest the entrant.
What the Want to Read list does is link the reader to your book. They haven't read it or rated it, and many people have thousands of books on these lists, but it means that when you post news about the book, or list another giveaway in the future, they will get notified about it in their timeline.
Think of it like a gentle nudge. Maybe they spot your cover and like it, so they enter the giveaway. They don't win and forget about it, but a few months later you offer another giveaway. Goodreads will tell that reader about the new giveaway and 'nudge' them into thinking about your book again.
Maybe, just maybe, it'll be enough encouragement to get them to buy it this time around. If nothing else, they remain linked to it.
You have to mail the books on your own, but giving away paperbacks is free.
Unlike Amazon, these readers are more invested in reading your book than if they got your book from an Amazon giveaway. You won't get reviews on a one-for-one basis, but I've consistently gotten about one review for every three books given away. I've also managed to make lifelong fans and friends from the giveaways, because these are usually people who love to read.
You can give books away, but you will get a lot more traction on giving away gift cards or other prizes. The thing is, you want people to buy your books, not just win a free copy, and the hope of 'winning' a copy might actually turn people away.
Big ticket products are also great for promotion, but to help with international distribution you can promote a big item, but give the prize as a gift card so they can 'buy' the item. Not all countries will have the item you are trying to promote.
This can easily break the bank if you aren't careful. My advice would be to find other like-minded authors and work together on something like this.
It is incredibly easy to build a giveaway and share it, though making it go viral is something else entirely. You generally need to pay to have images and more options to signup, but you can certainly run a free giveaway without much effort.
This is much like gleam.io. Both companies do a great job of giving away products and making it easy for people to enter. If you are at the stage that you want to give stuff away, then research both companies and decide if this is the route you want to go.
Don't put only half effort into it, though, because if you aren't able to gain a lot of traction it can be a costly venture with very little gain.