It's the middle of August, which means one thing: it's really hot outside, and the weather has probably never been so humid. Falling asleep without something that will keep you cool simply isn't an option. Not only will you find it a lot harder to actually get some rest (because you'll be so hot), but you also just won't be comfortable at all. If you don't have access to an air conditioner, sleeping can be a big problem during hot summer months like this one... and that's where fans come in. There are plenty of benefits of sleeping with a fan on that will make you want to invest in a good one.
As with anything else, there are also some negative effects to sleeping with a fan. Because of the way fans move air, if it's very hot outside, it could actually dry so much sweat off of you that it leaves you feeling a little dehydrated. Fans can also kick up dust and other allergens around the room, swirling them into the air and then eventually into your face, where they get into your nasal passage and can potentially cause a cold or annoying allergies. If you have a lot of allergies, you probably want to talk to a doctor before committing to sleeping with a fan.
Otherwise, though, if you're really hot and need something to cool you down, or need an extra boost of air even fi you have an air conditioner, a fan will do the trick.