This is a really good function to know about as it solves a common conundrum.

Oftentimes in a list form you'll get a situation where you have something like (All follows in pseudocode/pseudohtml)

[-- BEGIN block --]
{item1} {item2} {currency}{price}
[-- END block --]
{currency}{total}

Now, you'd itterate that block by doing something like
foreach ($items as $item) {
$template->setvariable(array('item1'=>'$item[item1],'item2'=>$item[item2],'currency'=>'$','price'=>$item[price]))
$template->parseBlock('block')
}

Now if you do a setvariable to get the currency in the currency/total, it'll also mark a new block and you end up with an empty block line.

HOWEVER if you use the setGlobalVariable, it wont unempty the block, thus leaving the table as it should