It lowers the resistance by about 5g. or so. It also helps stabilize the key. On the HHKB, it doesn't really do much, so I take it out for the lower resistance. On the Realforce, it gives the key a more solid feel when you press it. I like the more solid feel on the Realforce, so I leave it in.
Keep in mind, this isn't the spring under the dome, it's a seperate spring right under the key cap.