"... any organisation should employ on merit only, not to make up some arbitrary number".
I could hardly agree more, but there has always been a tendency for those in the positions to appoint people tend to choose others similar to themselves. I am not saying this is necessarily wrong, but there is often more than one candidate who is appointable among a group of very similar abilities. It does no harm to choose the person who is slightly different to the rest, and can bring benefits. The existing workforce are usually able to adapt to having a woman amongst them, or a person of a different religion or ethnicity; if they aren't, are they really the best model for choosing future employees?
A new perspective, however, can bring untold benefits. Having a woman in the team might have prevented Gillette, for example, thinking it was a good idea to charge more for razors simply because they are pink.
Women's Day, however, is about celebrating women's achievements, oftentimes overlooked. There was a round on University Challenge recently about a newspaper printing belated tributes to people whose deaths had previously been ignored by that 'paper simply because they were female. Ada Lovelace was the starter for ten.
Similar stories are now being told in films such as On the Basis of Sex or A Private War, and the importance of Clare Hollingsworth's contribution to war reporting or Rosalind Franklin's in the mapping of DNA.
I am sure some will find this objectionable, but tough. Live with it.