Many parents think that lower LATCH connectors will help restrain their boostered children in a crash. The purpose of LATCH on a belt-positioning booster is to secure it to the vehicle so it doesn’t become a projectile in a crash when the child isn’t using it and so it’s more stable for the child when they climb in and out of it. The seat belt is what restrains a child when they use a booster seat.
Using LATCH with boosters can be confusing because on harnessed carseats, LATCH has a weight limit for its use. It varies by carseat, because the more a carseat weighs, the lower the LATCH weight limit. However, flexible strap LATCH (the kind you find on most carseats and boosters) is definitely a convenience feature when it comes to booster seats (check your all-in-one manual to see if it can be secured with LATCH when in booster mode). There are some studies that suggest boosters with rigid LATCH may have better outcomes in side impact crashes, but flexible strap LATCH keeps the seat belt in place better on dummies. It’s baffling for sure!
Bottom line, if your booster has LATCH and you can use it in the position you want in your vehicle, use LATCH to keep the booster secured. If not, don’t sweat it, but be sure to have your child buckle the booster in so it doesn’t become a projectile while you’re driving without your kid in the car.