Random card rewards can easily be cards that you would have either not encountered or failed to encounter. It gives you at least another choice, even if it's not a better one. Some cards are pretty weak at the beginning but are much more useful in later chapters. Also, the random card can easily be one that was off limits when you initially built your deck.
Also, random items are automatically acquired (you don't need a check for them), so you could potentially get an item that would have been hard for you to get normally. Holy Candle (Wis 10 check) is one I can think of off the top of my head.
It isn't always going to help you, but it could potentially.
The odds of that item being one you would want to keep in you deck are the same odds that an item you encounter in the scenario would be one you would want to keep. 1 out of how ever many are in the box. And as zeroth_hour said, you don't have to roll to acquire it.
Yeah, sometimes its something you don't want, but sometimes it is something you do want. (And sometimes its Holy Candle or Crown of Charisma and then you'll think its the best reward ever given.)
Not every reward can be a feat check, or else you'd have all your check boxes full halfway through adventure deck 2. So they could give you nothing other than a congratulations, but instead they give you a little reward.
Also, remember that if you're playing with more than one character, even if the random reward you get isn't something YOUR character wants, the other characters might, and they get it for free. I often play with 4 players and the extra rewards can really be nice in that setting. With one or two characters, it often feels pretty "meh", but sometimes you get lucky.
I'm obviously an extreme beginner here, so please excuse the ignorance this question will undoubtedly show, but what prevents one from simply putting coolest item cards (the Holy Candle or the Crown of Charisma, to cite the examples used above) in one's deck to begin with? I didn't put them in because I just used the cards suggested by the quick-start guide, but now that I know how awesome they are, what stops me from just putting them in my deck the next time I build a character?
Your initial deck must be composed of cards with the "Basic" trait.
Note that some of the suggestions in the printing of the rulebook are mistakes on this front; ie Kyra can't start with Half-Plate and should instead start with Chainmail.
I'm obviously an extreme beginner here, so please excuse the ignorance this question will undoubtedly show, but what prevents one from simply putting coolest item cards (the Holy Candle or the Crown of Charisma, to cite the examples used above) in one's deck to begin with? I didn't put them in because I just used the cards suggested by the quick-start guide, but now that I know how awesome they are, what stops me from just putting them in my deck the next time I build a character?
Your grudging respect for the rules is all you really have to stop you from doing that. As has been mentioned, starting decks (assuming you're starting at the beginning of either Chapter 0 or Chpater 1) should be composed only of cards that have the "basic" trait. Many of these are not very good, so you'll want to acquire better cards through exploration and by getting the card rewards at the end of various scenarios. That being said, I'm sure plenty of people cheat and put whatever they want in their deck (and then complain that the game is too easy).
Holy Candle and Crown of Charisma are items, so there is also a chance they will be encountered as part of a location. It is random of course, but it does happen. So make sure you shuffle up that item card stack really well before setting up the locations.
The same is true of all the cards except Loot cards. Loot cards you only get if a scenario or adventure reward gives them to you. Everything else though is encounter-able in a location.
There are a couple of videos on youtube of people playing. Don't take them as gospel truth, because they sometimes have mistakes, but it can be helpful. There is a link to a list of them on the main PACG page here.
And welcome to the game. It is totally awesome. It does take a few times playing (at least for me it did) to grasp how the rules work, but stick with it. Its totally worth it. And don't hesitate to ask a question if you have one.