I believe the actual reason behind them is the following
The other dice traditionally used in a game of D&D (d4, d6, d8, d10, d12, and percentile dice) don't show up in the main set
Okay, so considering there is a bunch more stuff I haven't even looked through yet, I can pay $300 for this collection." Even though there may be a bunch of bulk, or more binders full of rares, I pretty much stop looking though the cards at this point
Sometimes the card is flavorful, or has a particular effect you need, or matches your theme
Placing a number of +1/+1 counters equal to the amount of mana produced by a mana ability can get out of hand quickly, especially for cards that look at the number of Elves you control like Priest of Titania or Elvish Archdruid
Bodyguard or Ready effects could be helpful, but also consider Mauis Fish Hook or even Longboat