What is the benefit of the Overload mechanic?
Why use cards with overload?
Cards with Overload usually gain you a big tempo swing when you use them. Popular examples are Lighting Bolt, Lava Burst, Doomhammer or Totem Golem. Especially those cards are very common in aggro/burst shaman decks.
The overload mechanic allows some very explosive turns, which is what aggro decks rely on. Also the overload does not matter when you burn your opponent down to zero hp with a combination of Lighting Bolt, Lava Burst and Crackle.
Also keep in mind that you can use Lava Shock to unlock overloaded mana crystals, which allows some devastating turns.
Advantages
- Allows big tempo swings
- Great burst
- More overall card value
- Synergy with cards like Tunnel Trogg or Unbound Elemental
Disadvantages
- Weak followup turn, opportunity for your opponent to make a big play when you are overloaded
- Limiting your options (you don't want to overload on turn 5 when you have a Fire Elemental ready for turn 6)
- Sometimes hard to curve out correctly
The Overload mechanic allows you to gain a tempo advantage by playing a card 1 or more turns earlier than an equally balanced card without the Overload mechanic could be played on. Or similarly, you can play the card in conjunction with other cards earlier due to its lower mana cost.
Think of it as "borrowing" mana crystals from your next turn to do something on an earlier turn.
Compare the Shaman card Lightning Bolt to the Warlock card Darkbomb. At a high level these cards are equal. Both deal 3 damage for the cost of 2 mana. The different of course is that with Lightning Bolt you can spread the cost over two turns, allowing you to play it sooner than you might otherwise be able to.
Just plain focusing the question without concentrating why one should use this (since others did this allready):
Is there any benefit at all to playing a card which reduces the available mana for the next turn?
Overload by it self isn't thought to be any kind of benefit. It is an dissadvantage. But its (ignoring cards synergyzing with overload) a feature making the otherwise much too cheap cards compared to its value ballanced again.