Why is there potato in meatballs?
With absolutely no citation or evidence to back up my claim ;-) I'm going to say that the reasoning is two- if not three-fold…. or maybe more...


It's cheaper. It adds bulk.
It helps the meatball stay whole; a binder. Similar reasoning as breadcrumbs &amp;/or egg, but see 3.
The binder is needed because the EU allows the meat producer to add 10% water to the meat before it's even completed the initial butchery stage[1]. Therefore you need some 'soak-up' to prevent the water being squeezed out as soon as you start to heat the product.
It helps the texture; stops it becoming too chewy. A mouth-feel thing, same as you might make 50% beef/50% pork meatballs. 100% beef can be a bit heavy.


from comments  


There's no chance of gluten intolerance issues with potato.


…and a late 


It's very probably traditional. Burgers get breadcrumbs, meatballs get potato. Why or why not; because it's always been done that way, which sometimes defies all reason.


[1] I personally consider this a cardinal sin &amp; were I ever to miraculously become 'President of Europe' this practise would become a capital offence ;)