A normal computer is usually constantly writing little bits and pieces of data to disk. But data on the disk might accidentally remain on the disk even if it's not intended. Then that data could be read later by someone else who is spying on VPN users .

There's also a common assumption that data on disk storage may leave behind remnants even after it's been overwritten. (Magnetic disks may leave behind some magnetic signatures. Flash drives will stop using sectors that are worn out, potentially leaving data there.) And state actors like NSA might have some capability to recover this ghost data if they get a hold of the actual drives.

There's a general understanding that data on RAM is irrevocably destroyed within a short time after the device loses power. So attacks on RAM data have to occur in real time while the data is in use. (There may be some attacks that preserve RAM after power down using low temperatures and liquid nitrogen).