the first and easiest is they are the only thing that does that job in the entire army! quite easy to guess what this might be since legion have 2 choices really in this category
the first is spell martyrs our 1pt exploding arc nodes we don't have any other alternatives so that's it lol and the same for the humble shepherd it does 1 job and does it well lol also strider death stalkers for clearing infantry as a 4pt solo that ignores tough its hard not to take them in every list !
the second reason I think is 1 that 1 that defies normal comprehension
and the koolest model I own is the archangel and not many would disagree that it is the best legion model and one of the best that PP produce a giant dragon what's not 2 like! and for taking an awesome model the hefty price tag is that its a huge PILE of dogcrap! whilst PP said they would turn gargants up to 11 they mainly jus doubled the points and added a few extra boxes and messed around with the animus. OK it's the only huge based model that can move out of activation giving it a 10" move the down side is its 11/18 like a standard heavy and fury4 like a standard heavy for what is now double the cost of 2 regular heavies its simple not worth the hassle although it has its places where u can buff it up with stat increases it makes it more tempting abby2 lylyth2/3 kyrssa are some warlocks that can make it work
this is probably the main reason we have auto includes its those models that are plain awesome and better than anything else you can choose for me (mainly coz I now own these models lol) proteus and typhon are expensive but better than their respective alternatives as they bring more to the table this can also occur with warlocks as well
their is a downside to auto includes in that they can make your lists restrictive if u use up points on these models!