In a nutshell - it's because there's milk in some potato crisps!  

Animal products are in so much of what we buy and consume - even if we think we are managing to avoid them.  

Food and drinks mostly have labels, as do products.  But would you check the label for a bag of crisps?!  You need to be on the ball as there are so many unsuspecting places for animal products to be used in things -  especially in my industry.   

I completely understand why a client may want to make it known to those on a team specifying for a build project.  I also think that may feel like an uncomfortable thing to do for them - as that team may have no awareness that animal products  are used - let alone that sheep's wool commonly used in insulation is something a vegan may want to avoid or that animal glues are used widely, still.   People still have a block that vegan is all about diet and not a consciousness about how we live as well. 

Even though I've been working in this area for many years and I'm familiar with what goes on - I'm learning of more all the time.  I recommend fellow vegan interior designer Aline Dürr's book Vegan Interior Design.   I guess the clue was in the name but I was surprised to read that bone china still contains a large percentage of bone ash from pigs and cows.  You just think these things would have been superseded over all these years.  To me it just sound out-moded.

Even if someone is not vegan - these are still things they might want to avoid.  I've recently completed a fantastic and informative set of courses on Healthy Materials and Sustainable Building devised by the Healthy Materials Lab at Parsons School of Design.  Chemicals, wellbeing and sustainability are all interconnected.  Chemical use in interiors products mean the need for toxicity testing of the products - and that means animal testing.  Sometimes this happens once and the findings are shared between companies.  But new chemicals / combinations mean new testing. 

So chemical use should be something we should be cautious of for many reasons if health, environment and animals are important to you.