Because it takes unnecessary write cycles off your NAND cells, Flash memory cells have a limited amount of times they can be overwritten before they wear out, 50.000.000 times on most modern drives, after which they appear as a bad block would on a HDD, under normal daily usage it'd probably take decades before a large enough number of these cells wear out for the drive not to function properly, but if you do lots of reformats and/or defrags on a regular basis it will reduce the lifespan of the drive.
An explanation of what is does is here: http://www.piriform.com/docs/ccleaner/using-ccleaner/wiping-free-disk-space
It is also completely useless in the case of SSD's because what makes recovering deleted files on a harddrive possible is the magnetic nature of the drive itself. This is better explained here: http://techgage.com/article/too_trim_when_ssd_data_recovery_is_impossible/
Edit, anort beat me to it.