'dft' is popularly used for discrete Fourier transform, which too is going to be a buzz word on this site. Let's just not use 'dft' as a tag and use full forms for both, density functional theory and discrete Fourier transform as tags.
I strongly encourage y'all to spell out tags as much as possible unless it's a dead-standard term as an abbreviation even outside your field... Like... scuba.
I'm happy to synonymize any of these to the master for you as I've done in this case.
As to why - well, the site should be something that people of all skill levels can get to know and feel comfortable using. If you're relying on shorthand, you're excluding people who might be newer to your field. It's also easier to see at a glance what it means. With abbreviations/initialisms there's always the chance that there will be need for disambiguation - that's much less likely if things are spelled out.