A baby is a very young human being who can barely speak let alone walk. The baby stage is considered to go from 0 to 15 months, approximately.
Fundamentally, these are very young children who cannot hold their posture for very long, who cannot walk unaided, and have not yet learned to speak (in the majority of cases). They are extremely vulnerable children and we need to protect them the most because of their delicate condition. Moreover, we should take into account how fragile they are and the fact that their heads are much larger in proportion to their bodies. For this reason, protecting their most vulnerable areas is crucially important and this is why they should travel in a rear-facing seat.
If we take a look at the i-Size approval standard (ECE R129), it guarantees that all rear-facing seats can be used until the child is a minimum of 15 months old. By doing so it encourages the use of rear-facing child seats, which is a much safer position for children when traveling than a rear-facing seat.
In Spain, children must travel in a rear-facing seat until they are at least 15 months old, and it is advisable for them to do so until they are 4 years old (both R44-04 approved seats, Groups 0 and 0+, and R-129 approved seats are rear-facing up to approximately this age). The main reasons for this safety recommendation are because children under four years old can suffer far more serious head and neck injuries since they do not have fully developed muscles.
Placing a baby in a forward-facing car seat is putting them at serious risk of injury in a crash. Therefore, it is advisable to purchase a good rear-facing seat and to keep the child in it for as long as possible.