you can trade them for strong magic or there used with some weapons...Armored Spider gives a Fire Bullet spell( 4got name)

@dom:

http://demonssouls.wikidot.com/bosses

Click on the Demon's Soul you want to know about, and it will tell you what it can be redeemed for. It lists all the bosses, so there's potential spoilers there.

I've only learned 1 spell and I'm almost done with my first play through the game (just need to beat the boss of 4-4 and endgame). So no, spells are definitely not needed to beat the game. I'm using a Dragon Longsword +3 and a Lava Bow, and the only part of the game that really gave me a hard time was the boss of 2-2, the Flamelurker.

You can always get the souls again on the next play through anyways, so it's not a big deal.

The game becomes actually harder every New Game on the same saved file. There's no 1 shotting bosses in this game lol.

As for spells, I managed to finished the first game with no spell. I like melee type characters. Once I started NG+ and above, I got spells and realised that they are overpowered especially in PvP.

So don't worry if you consumed them in the first game.

do you, AT LEAST, create a new character in new game + ?

or do you use the same ?

it's a lot more challenging, and when you start again with a NG+ you'll see new bosses, new items, and new black phantoms/white phantoms.

also, you don't have a defined class when you already start the NG+....after some time playing the game you can be every class in just one guy and this is why you start a NG+, because the bosses gives you more souls, more xp, more bla bla.

http://www.demons-souls.com/wiki/

I have found different explanations for defeating certain bosses and other stuff, so you may need to search around in order to find what works for you or your character build. Also IGN's walkthrough is fairly spartan, but it can help as well. 

An important thing to understand is World Tendency and Character Tendency. Certain major demons will only appear if WT is completely black or completely white. Also CT will dictate things as well. But you can still beat the main game without triggering all of the events.

In Demon's Souls don't get too caught up in leveling up, as you progress souls will become more abundant. It is more important to upgrade your weapons correctly. In order to do that effectively you need to learn about the Crystal Lizards.

If you need places to farm, let me know.

Easily the deepest most complex game ever released to console.

I think this is my game of the year.

It is possible to trigger all events and all major demons in your first playthrough.

And only NG+ scales up, NG++ remains the same 

From what I've read, only NG+ significantly ramps up the difficulty.  Subsequent playthroughs do up the difficulty, just not as sharply as the 2nd playthrough.

Thanks for all the comments, guys!

I have still not managed to kill even one Crystal Lizard, but I have managed to level my Bastard Sword up to +7 and it's pretty badass. I more or less breezed through 2-2 with that sword. (My guy is also Soul Level 32).

I also wanted to say, YES, this game IS that interesting. If I didn't have a job, a wife, and a 2 year old to raise, I would play this game all day every day. I could just eat, sleep, go to the bathroom, and play Demon's Souls. I love it that much. The funny part is that when I first got it, I almost traded it in for Dragon Age because I was so incredibly frustrated with it. I have Dragon Age now too, but it mostly sits on my shelf untouched.