It's for presentation.
There are many way to cut a carrot [of course].
I would only ever cut straight across for a traditional 'English stew'.
On the bias for something more 'Continental' [I know, these descriptions are vague] for a rustic vibe. For 'extra rustic' I bias cut one at 45° then the next cut straight, to make 'triangles', & the same for such as chorizo. I don't know why anyone would assume a French farmer's wife [antiquated example] cannot cut straight, but it's a perception.
For anything I was preparing the carrots as a side dish, then I always baton/allumette or occasionally jullienne. I don't throw out the edges [see below], I cut them similarly. Personally, I think this brings more of the flavour out. The flavour overall seems sweeter; perhaps because of the difference in balance between carrot outer & inner.
Leith's cooking school has nice examples of each type - though I've not seen turned carrots or potatoes used in a restaurant since the 80s ;)
btw, just because you cut most of it on the bias doesn't actually mean you have to throw away the end bits. It's not Masterchef, you're not going to lose points for having two pieces out of every ten that aren't identical ;)