I'd love to make the joke "when you find that a crazy girl with knife is so hot that you can't help but want to have holes in your knife so you can fuck it".
Sadly, I also want to know. If we still used blades as proper weapons, I'd say it makes the blade lighter overall, and nowadays metal industry allows it to still be resistant besides the many holes in it. But I doubt this is the reason, since as you can see, this argument mixes modern metal industry with the need of medieval like weapons.
And because of that, I searched for it. Here are the reasons:
- Put a piece of silver in it to fight werewolves.
- Opening bottles and alike(it depends on the hole format).
- Make more noise when you slash it in the air, so it can look/sound about 20% cooler.
- It's really for making it lighter. It may also help to use less metal (and save costs).
- For looks.
- They were once used by the cops and the army, but not as weapons, but as targets.
- If it's on the handle, it helps ventilation.
- To look through the Eye of Thundera.
- To hang it in a hook (yeah, some knives, like cleaves (obese knives) are cool to put on it).
- Attach magical gems to increase stats and unlock skills.
- Less area for food to stick.
- Social Darwinism against people with trypophobia.
- Attach things on it, depending on the use.
- Inducing anxious people to want to break it by forcing the weaker part, so they will have to buy a new one.
- Put one of your fingers (or another thing) through it (depends on the hole format and it's localization).