I've known for ages about the enzyme called telomerase, & I've known why it's important - but I have to confess that I've never actually thought particularly hard about why we need it. 'Why' being in the practical, 'what's going on in the cell?' sense. Lazy thinking on my part, I guess. But today I found out, & learned something new :-)
Telomerase is an enzyme that comes into play in some cells during DNA replication. This complex process requires that a double strand of DNA be unwound & a copy made of each strand. (There's a good tutorial & animation of the process here.) A number of enzymes are involved in this, but for some reason life has only ever evolved one form of the enzyme (polymerase) that actually carries out the copying part of the task. And it copies DNA in one direction only - a difficulty overcome by the fact that one strand is copied in a continuous piece, while the other is copied in smaller chunks that are later stitched together by yet another enzyme. (Have another look at that tutorial - and there's another nice animation here. There are some wonderful educational films & animations on YouTube!)
One of the consequences of this is that each time a DNA strand is copied, it will get a little shorter at one end. Obviously if this meant that functioning genes would be lost, we'd have a problem; the evolutionary solution to this problem is telomeres - long non-coding repeating sequences that can be shortened many times before the cell's functions are affected. However, there eventually comes a time when the cell can't replicate any longer - this means that somatic (body) cells don't have the ability to go on replicating forever. But the cells that produce gametes - the 'germ cells' - have active telomerase, which restores the sections of telomeres lost when these cells divide. (Unfortunately, cancer cells gain the ability to express telomerase, thus making themselves effectively immortal - some lineages of cancer cells have been grown in the lab for decades, far longer than normal somatic cells can continue to divide.)
The discovery of the roles and functioning of telomeres and telomerase was recognised by this year's Nobel Prize in Physiology or Medicine. And you can read an excellent explanation of the whole telomere/telomerase thing on Larry Moran's blog Sandwalk.