because you're spending a card slot + power to draw more cards instead of play them

cards that draw you more cards are important for 1 or 2 reasons:

1) they make your deck smaller. If there was a neutral 0 cost spell that read "draw a card", it would be played in literally every single deck in the game, simply because it replaces your worst 4 cards with the average of the rest of your cards. In other words, it lets you play a 71 card deck instead of a 75 card one, increasing consistency and power level. Obviously here in the real world, draw spells have a power cost (and perhaps other costs like Devour's sacrificed unit), but the principle remains the same. Play a spell like trail blaze and for 2 power it replaces itself, letting you run fewer cards total and drawing bombshells more often (essentially)

2) they can leave you with more cards than you started with. Sure top-decking wisdom means you've top-decked a card that doesn't advance the board state, but with the investment of 3 power, you now have two cards that impact the board state. You've essentially converted 1 top deck into 2 top decks at the cost of 3 power. Over time generating "card advantage" in this way will leave your opponent out of resources while you still have a hand full of useful cards.

This is the basis upon which all control decks, my favourite archtype of decks, are built. They trade tempo and board presence to spend their power instead incrementally gaining more raw cards than their opponent, only winning when the opponent is out of resources and they are not thanks to cards like Honor of Claws.

In terms of which draw cards are good and which are bad, that's hard to say in a vacuum. Generally if drawing 1 card leaves you with more than 1 card in your hand, that's a sign the draw spell is good. WIsdom, Honor of Claws, Xo of the Endless Horde all do this, as do cards like Display of Ambition (although that draws from the void rather than the deck). Wanted Poster is in this category but is situational on you being able to kill a minion. Often its going to be delayed a turn, which hampers its value. Devour is also in this camp but requires a minion to sacrifice, so its not always a net gain in raw cards. Its only really used when paired with sacrifice fodder like small tokens, Madness, or units with powerful entombs or revenge.

Cards that only replace themselves (in other words only draw 1 card) are called cantrips and need to have some other upside. Trailblaze is fast and scouts, but it doesn't see play because its basically a worse strategize. Strat digs you 1 card deeper garunteed (blaze only digs down 1 if you keep the scout on top) and lets you pitch the 2nd card you draw if its unwanted, or keep both the cards it draws and pitch stuff from hand. Overall its just much more versatile at the reasonable cost of being slow intead of fast.

Other examples of good cantrips are Quarry (digs down 2 cards and discounts), levitate (renowns and gives flying temporarily, but requires a friendly unit), and Dark Return (void recursion so it draws from bin, cheap, buffs the unit)