Why they would buy it - They are out of anything else to buy.
Why they would use it - Well.. Basically, they wouldn't. Technically there are some exceptions. Mindblight raid in SoD expansion had a good use for a wizard burning this glyph as he does not summon, so you could have someone out of range hold aggro and no one actually tanked it or Masked Invaders raid in Underfoot the final boss use to be kiteable (He can no longer be snared, though he still doesn't summon). In practice, it was usually better to have an actual tank hold aggro out of melee range, but wizards were capable of it, and this glyph would certainly help.
But all the glyphs in the special tab are not class specific as Amarth said. That's the real answer.