As always it depends
what you want to do with it, there are lots of things they can do (growing daily) - a few suggestions with comments:
Play your MP3s/other digital musicTo do this, you upload them onto an online space that Amazon gives you. There are tiers of storage, starting at free and going up depending on how much storage you need. But, unless you are serious audiophiles, chances are the free space will be enough.
play radio stations for youMuch like BBC iPlayer Radio app for the phone. This is free for all the radio stations I've tried. There may be some you pay for though.
order stuff from amazonNo cost as such, obviously you have to pay for what you order, and if you want to use Amazon prime delivery, this carries a cost.
play music from Amazon's own music libraryThis is like Spotify and similar services. It's a bit like when you upload your music, except that you have access to millions of tracks Amazon already have. Cost is £10pcm. However, it's worth checking if you have mini me's or other family that there isn't already a subscription somewhere as you can upgrade to a family plan for an additional £5pcm.
set alarms, ask about the weather etc etcFree, other than electricity costs
home automationBeyond the scope of most users, but generally the software is free (a platform/methodology called IFTTT is often used to get different bits to talk to one another), but obviously you have to buy the hardware.
Hope that's a reasonable starter for you