Just an example I come up with. 

Someone installs a keylogger on your PC. You log in with the password that gives you the highest permissions. Now the 'hacker' has this password and can change the password of your account. You are locked out forever.

It's just bad practice to log in this way because it's not necessary at all. 

Because you are more familiar with the platform I guess it's alright for you to log in with your 'active key'. This will give you all the permissions that you will ever need. And if this one gets stolen then you can always fix things with your owner key or the password they gave you when you created your account. 

I hope this was clear. If not. please just ask.

*EDIT*

Just a sidenote. if you use the active key and it's stolen then any Liquid Steem / Steem dollars can be transferred.