Back is best for sleeping babies, but time on the front and time upright when awake are both good for babies' development, too. The varied positions help to maintain a good head shape because gravitational forces are distributed differently in the different positions. It may also help to prepare a baby to move away from any obstruction and so protect their airways should they need to.
Babies need to practice lifting their head and turning their head from side to side until they can coordinate a high lift and wide turn. It is this combination of skills that seems to add safety. It means a baby is more able to move out of trouble and protect their airways if problems arise.
The effort of bobbing and nuzzling is quite a work out for a young baby. They can tire quite quickly. They need to be supervised in case they fall asleep. Evidence from SIDS research is strong and clear that tummy time is risky for sleeping babies, especially if it is the first time they have slept on their tummy, or if they are not very experienced at combining head turning and lifting.
When babies do fall asleep they need to be positioned on their back. This position gives the best protection from sudden infant death syndrome.