Right now, there isn’t a huge deal between them – maybe there never will be.
However, the point is to give the browser-makers the ability to do something special with it, if they want.
Think about <input type="number"> on cellphones, bringing up number pads, or type="email" bringing up a special version of the keyboard, with @ and .com and the rest available.
On a cellphone, search could bring up an internal search applet, if they wanted.
On the other side, it helps current devs with css.
input[type=search]:after { content : url("magnifying-glass.gif"); }