Rear-facing car seats often take up more room into the front seats than parents expect, or want them to.
With the handle in the “travel” position, the carrier becomes several inches wider at the top – which means that the car seat is less likely to fit properly in the center of the back seat because it will often be too wide to nestle in the gap between the two front seats without forcing the driver & passenger to push their seats up.
With the handle in the “travel” position, the carrier becomes several inches deeper – meaning that it takes up more room into the front seat, extra room that may prevent the car seat from fitting behind a taller adult in the front seat.
Therefore, the most versatile infant car seat allows the handle to be in the carry or anti-rebound position as these two positions will make it such that the car seat takes up as little room in the vehicle as possible. With the handle up, the carrier is more likely to fit securely in the center of the back seat, nestled between the two front seats, while still allowing the driver and front passenger enough room.
The image below shows how much room various infant seats take up into the front seat (all were measured with the handle in the position that the manufacturer allowed that took up the least amount of room into the front seat).