What business does not want to increase revenue? It is important to gain as much exposure as possible for your brand and that means looking outside of your own website. Selling directly from your own website is of course the best option – your margin is greatest that way. However some of the other sales channels have the advantage of huge traffic volume which is worth taking advantage of.
The main advantage of using alternative sales channels is simply huge traffic of buyers. There is a second important one though, and that is trust. People like buying on Amazon and other major online stores as they know the name, probably have an account already set up and they have dealt with them before. This creates a massive amount of trust which in turn increases the chance of a visitor converting to a sale.
The downsides are the strict control on listing formats and what you can say to sell your product. The other major downside are the fees and commissions you will have to pay out. In terms of Amazon this usually works out at 20-30% of the selling price, so unless you have a good margin, it can be problematic.
In my experience these sales channels can be well worth testing. Some products are more suited than others. It is important to give them a decent length test as well – I have found it takes months for the sales volume to build up, but it can build to a level which becomes well worth the effort it took to set it up.