Why do we need to rectify or amplify the current/voltage?
A power company wants to generate electricity, but they don't want to waste it on power transmission. 

Power loss in a wire is given by 
$$P = I^2R$$

but you can also calculate power as 
$$ P = IV$$

where I is current, V is voltage, and R is resistance. 

So power companies use very high voltages, as this lets them use very low currents, which means (relatively) little power loss in the transmission lines. 

You don't want to use very thick insulation on your electrical cables in your house, though, which is what's required to prevent you from getting shocked by the 100,000 volts the power company produces. You would like to get the voltage down to, maybe, 120V or 220V. 

So the power company makes alternating current, or AC power, because this creates a time-varying magnetic field around the wire. If you get a bunch of this wire together, then you can use that magnetic field to induce electricity in another bunch of wire. 

The neat thing is that the voltage output of that cluster of wire, called a "transformer", is related to the ratio of input to output turns. This means you can now step down from 100,000 volts to 120 volts. 

But, your computer is a digital device. It likes things to be "ON", at a constant voltage, or "OFF". If the voltage is time-varying, you don't get to have nice things like logical TRUE or "ON". So you would like to rectify the AC power into DC power. 

You do that with an arrangement of diodes called a bridge, which technically creates ripple DC, which you can smooth with capacitors to approximate a steady DC supply. 

Then your computer works. 

Also, you might want to shape electricity to make interesting shapes with it, like the PAL signal to make images on TV, or to reproduce audio, or any number of other amazing things that you can do with analog electricity.