I'm not an expert but I think : The site you visit only sees the VPNs info. Which is how you maintain some anonymity while browsing. However, if your VPN keeps logs, then you can still be tracked, just at a different place. Some say they don't keep logs, and you'd have to trust that.

RAM is considered volatile memory, so each time the server turns off, it loses all data. This is compared to disk (hard drives of whatever type) which retain memory even if the server turns off.

In theory, this ram only server prevents them from keeping logs (like which user went where) since the server wouldn't even have a place to store it.

Edit: lustrums post is more accurate and has info that this doesn't prevent logging per se, but could prevent accidental logging. I.e. they can't hire a forensic computer specialist to parse through operating system logs to try to find info they didn't otherwise log elsewhere.

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).

If the computer is unplugged, there's nothing left on a hard drive to show what state it was in. This means nobody malicious can physically remove their servers and gain information about customers.

No data is supposed to be readable after you shut it off. There are ways to restore it though but it’s still vastly better in leaving no trace.

It means that even though Mullvad already doesn’t log anything about their users activities, there is no persistent storage on the servers, so as soon as it is powered off or raided by The Agencies, there is absolutely nothing to retrieve from it.