Because, from what I've read for more than a year now, if the bank wants to foreclose, but it cannot deliver your original mortgage to the sheriff/bank, who cannot then present it to you--in exchange for the property you are being forced to vacate--in the original, with your original signature, when they come to evict you, you can refuse to move until they produce it.
Which, as the article I link to below, is not always as easy as you'd think:
Over 62 million mortgages are now held in the name of MERS, an electronic recording system devised by and for the convenience of the mortgage industry. A California bankruptcy court, following landmark cases in other jurisdictions, recently held that this electronic shortcut makes it impossible for banks to establish their ownership of property titles—and therefore to foreclose on mortgaged properties. The logical result could be 62 million homes that are foreclosure-proof.