i have seen 'band' tag for countless mp3 files. i am removing it, because i have no need for it (in most occasions, its the same as 'artist' tag).
but could you tell me why do you use (if you do so) this tag or wich program tags files this way?

Band is another method of adding "album artist" to your mp3's, a lot of software uses the band tag and rockbox (the free firmware for mp3 players) takes the band tag for it's own database.

I don't use it or know of any particular programs that do, but I suppose I could think of some relevant examples...

artist: Jimmy Buffett
band: Coral Reefer Band

artist: Bruce Springsteen
band: E Street Band

artist: Bob Seeger
band: Silver Bullet Band

artist: Tom Petty
band: The Heartbreakers

Most of the artists perform with these bands on all of their albums, but I know for a fact that a couple Tom Petty albums were in part without The Heartbreakers (Full Moon Fever and Wildflowers). Tagging them separately would keep your album list a little cleaner - just one "Tom Petty" node, not "Tom Petty", "Tom Petty and The Heartbreakers" and "Tom Petty & The Hearbreakers".

Also, with Jimmy Buffett, I believe the Coral Reefer Band have released a few albums without Jimmy. Tagging the band on the Jimmy Buffet albums could help you find all music including the Coral Reefer Band.

In the end, as all tagging issues, it's largely personal preference.

Since the BAND tag is generally used in place of ALBUM ARTIST in many programs, it would be used in the same way. For example, an album that has different artist listings (with featured guest artists on different tracks) that you want to group together under a single album artist when sorting and organizing:

Album = Cyclone (2007)
Album Artist / Band = Baby Bash

"Cyclone" Baby Bash ft. T-Pain & Mickael
"Supa chic" Baby Bash ft. Paula DeAnda
"What is it?" Baby Bash ft. Sean Kingston
"Mamacita" Baby Bash ft. Marcos Hernandez
"Dip with you" Baby Bash ft. Aundrea Fimbres
"As days go by (the love letter)" Baby Bash ft. Paula DeAnda
"Na na (the yummy song)" Baby Bash ft. Jean
"Don't stop" Baby Bash ft. Keith Sweat

Is album is by Baby Bash, but each track listed has it's own featured artist. I'm not one to put such info in the TITLE since I don't see "featuring..." being part of the track TITLE. Grouping everything under the ALBUM ARTIST tag in foobar2000 sorts everything in proper order. Using the BAND tag for my iPod does the same thing, since the Rockbox software does not use ALBUM ARTIST.

The same use would be for Soundtracks and other kinds of compilation albums where each track, or most tracks, have different artists. I list "Various Artists" in the BAND tag.

After all that, I don't use the BAND tag to list actual band names, I just use it as I need it. Band information is placed in the PERFORMER tag, if anywhere at all.

Edit: spelling