Ok. But what happens if, for example, this computer stops working? What do I have to do so I'll be able to sign the app on another computer so I'll be able to continue posting updates on Google Play?
Well 
grigrizljac as long as your app is still present in the control center of the intel app dev center(here) and as long as you use the same identifiers in intelxdk and you just increase the version number then theoretically your app should be singed with the same key no matter on what computer/device you are, as you would be using the same account to log in.
But if the app should get deleted from there, or that server falls, or gets hacked, or there is a power outage, etc. then you might be screwed.
That's why it's important, in my opinion, to sign the app yourself, because then you are in control of the security of the key, and you can do whatever you want to make sure it's kept safe; eg. you can save it on different hdds, you can put it on a bunch of cds/dvds, some sticks, heck it's so small you could even save it on a floppy disk....(don't get me wrong you'd have to be crazy to still be using floppy's in this day and age, but hey you can do it if you so wish:D)
And of course doing it yourself helps for other reasons, for example because it gives you control over other settings as well, like how long the key is valid for(because once the key expires your app is done), and I have no idea how long a key created automaticaly by intelxdk(or other services) is valid for. When I create my own key I usually set it to be valid for 10.000 days. Which is about 27 years, so I think I'm covered, loool.
Olso I have no idea how you would integrate google game services(for leaderboards and stuff) with an automatic key. Because you need the SHA1, and I have no idea how you can get that if you don't physically own the key...