ISOFIX connects the car seat directly to fixed points built into your car, removing the seatbelt from the equation entirely (some seats still use a top tether or support leg alongside it). Misrouted seatbelts are one of the most common real-world car seat fitting errors, so ISOFIX meaningfully reduces the chance of a mistake.
The trade-off is cost and flexibility: ISOFIX bases are usually sold separately from the seat itself, and a base is specific to one seat range, so if you switch car seat brands later you may need a new base too.
Belt-fitted seats are lighter, cheaper, and move between cars more easily (useful for grandparents’ cars or taxis), but only if the belt is threaded correctly every single time — which is exactly the risk ISOFIX is designed to remove.