You use that 12-24 word seed as a backup. Let's say, if you lose access to your computer, you grab your new computer(or phone), install a reputable(and open source) wallet app, enter in the 12-24 words, then boom shakalaka. You now have access to your funds.
So that assumes that I have my Bitcoins in an online account ?? Does this work if I have my Bitcoins on a USB drive

No worries, I trust you guys, no one here would try to use this for a bad purpose.I hope you are joking with your last line?
Trust nobody on this forum [no offence to anyone in particular] but accounts get
hacked here from time to time. We have a "Trust" rating for everyone, some users
have gigantic positive trust others like myself have a small amount, others have
none and then some have Negative Trust.
You got to treat those 12 words like they are the keys to life itself, nobody should
know them.