Mike Calligaro from the Microsoft Windows Mobile team has a blog entry that looks at benefits that persistent storage offers under Windows Mobile 5.0.
Earlier versions of the Microsoft mobile OS used RAM which places heavy burdens on the battery. The amount of power needed is linear with the amount of RAM. That is to say, 64M of RAM needs twice as much power to keep it running as 32M does. Flash ROM, on the other hand, burns pretty much the same amount of power regardless of how much you have. Did you know that 128M of RAM requires about 500mAh to stay powered for 72 hours? Fortunately, persistent memory will help alleviate this dependency on the battery. Now, running out of juice will not mean the loss of our data and programs.
Unfortunately, there is a downside. Flash memory is much slower than RAM. Expect your syncs and write operations to take a little longer.