They have user logins so that you can input/associate stuff like your tail number, aircraft performance data, navigation equipment code, color scheme, W&B data, etc. with your account so that you don't have to re-input/associate that data every time you want to plan a flight or file a plan. It's also handy to have a list of all your active and recent flight plans. It's just a convenience thing, from the user perspective. From their perspective, maybe they track the airports I show interest in and the plane types I file with to target the web ads they show me on their site, I dunno.
If I've ever gotten spam from flyplan.com, I've never noticed it. I have a pretty good filter that shields me from that stuff.