Updated: below is the company literature on why you should use a harness, however reading the below comments and links, looks like there is substantial evidence to the contrary.
I may be returning my harness! Marked this expired, seems like it would make a better forum topic
Cheapest I've found, normally around $69.00 (baby bunting) these also appear identical to babylove, same parent company?
The Protecta PLUS Harness features a frontal adjuster that allows a parent to thread the harness through the slots of an Expandable Booster Seat or Convertible Booster Seat. This provides a better fit on the child's shoulder all Safe-n-Sound tethered Boosters Seats.
Booster Seat - For use with compatible Booster Seats and Convertible Booster Seats for children from approx. 4 years of age until the child's shoulder's reach the Upper Shoulder Height Marker.
Vehicle Seat - For children who have outgrown their Booster Seat or Convertible Booster Seat approx. 7 years of age up to approx. 10 years of age when used with a lap only seat belt.
Maximum capacity - Suitable for children up to 36kgs. Suitable for use with a child on the vehicle seat until the child's shoulder's are above the top of the vehicle seat back or headrest. If the child is over 36kg then they can use the vehicle seat belt.
The Protecta PLUS Harness features a frontal adjuster that allows a parent to thread the harness through the slots of an Expandable Booster Seat or Convertible Booster Seat. This provides a better fit on the child's shoulder.
All Safe-n-Sound tethered Boosters Seats & Convertible Booster Seats are fitted with our patented VERSA TETHER catch which allows the connection of the Protecta PLUS harness using the single vehicle anchorage fitting. A Child Harness is suggested for the use with Booster Seats for improved upper torso support. Reduces injury associated with lap-only seat belts when used with just the car's seat. A Child Harness must be used if the booster seat is used with a lap-only seat belt. Suitable for use for all safe-n-Sound Booster Seats.
I spent a long time over this option - first trying to find out the difference between the Protecta vs Protecta Plus (Plus supports a heavier child), then whether I needed one at all. In my case, we have the superseded weight based Maxi Rider. I think either Protecta or Protecta Plus would've worked with our model, but in the end after seeing how this harness works with the lap-sash seatbelt and just using the car's seatbelt alone, I ended up skipping the Protecta harness completely.
This was also the cheapest I found, but if you're still on the fence, do your research, consider your child's age, height etc then decide. Do note a new booster is only around $150 or so and uses your car's seat belt (assuming not lap only seatbelt). Whether getting this harness works best with your existing convertible child seat or you wanted to prolong the same child seat for as long as you can (eg not convertible; too early to switch to new booster) may sway you in the end.