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.
How can you assure yourself that they will not take advantage of you? you have given them the opportunity to get the benefits they wanted from you, do you really think that your funds are still safe and secured? it's better if you listen to them because you might just end up feeling remorse if ever because you don't know the mindsets of other people particularly when it comes to something that can be beneficial to them.