2FA pops up on the computer i'm signing into, so whats the point?

This han't always happend. I used to only get the pop up for 2FA to my iPhone but recently it has started popping up on my mac when i am logging into apple or icloud. Thats not secure. Isn't the whole point of 2FA so someone can't log into apple from my computer? I don't know, maybe I don't understand now it suppose to work.