You need a first aid kit because they keep all your medical supplies together in one place, so if an emergency does occur, you’ll be able to act quickly. If you or a family member needs first aid, you aren’t going to feel like hunting around your house looking for bandages in the bathroom drawer, your desk for a pair of scissors, the medicine cabinet for antibiotic ointment, etc. A first aid kit is also portable, so if someone crashes their bike in the driveway or suffers a campfire burn in the backyard, you can simply grab the kit and be confident that you have everything you need.
If you’re out of your house on a hike, camping trip, soccer practice or scout outing, a first aid kit is even more important. Even relatively minor injuries can become serious issues if you have to wait until you reach a hospital or civilization to get treatment. This will also prevent you from having to improvise with unsanitary or inappropriate items that aren’t meant for treatment. A sterile bandage is going to be a lot more effective in the long run than a dirty handkerchief or sweaty T-shirt.
A first aid kit is also an important piece of an effective emergency or disaster kit, which is a larger, more comprehensive kit meant to prepare you for a weather or natural disaster. In this case, you may not have access to a hospital or ambulance for some time, making an effective first aid kit even more important to have on hand.