Most carriers have the ability to carry your little one on your front and on your back. There are also special back carriers with a frame, in which your child sits on a sort of seat. These are especially intended for longer walks.
A child can only be carried on your back when they have enough muscle strength in their back to sit independently. Otherwise, your child is not adequately supported on your back and may slump. (An exception are certain knots with a wrap that allow you to carry young babies on your back, but these require a lot of practice!).
A back carrier with a frame allows you to take your child with carrier off your back and let them sit in the carrier on the ground. There is also often room for all kinds of things to take with you. Handy on a hiking vacation! The disadvantage is that your child sits a little further from your back in this carrier. This increases the so-called 'moment' that your child's weight exerts on your shoulders. Carrying is therefore a bit heavier on your body. Furthermore, you can't just take these large carriers in your bag, so we don't see them often on the street.
In a soft baby carrier that you can use on your front and back, your child is also 'stuck to you' on your back, making it easier to carry.
But when should you switch from front to back carrying? This depends on what you feel comfortable with. As your child gets older (and heavier...), it may eventually become too heavy to carry them on your front for a long time. Your child can also become so long that it becomes difficult to look over them when they are on your front! Keep in mind that the minimum age for carrying a child on your back is 9 months.
Then you might want to try back carrying.
In the NIMBEL carrier, the moment you want to start back carrying will probably be later than with other carriers. The NIMBEL carrier has compact Exoframe bars, which means that your child's weight does not hang on your shoulders. This keeps it comfortable to carry on your front for longer. You can also wear the hip belt lower, so you can still look over your child's head. But the NIMBEL also has advantages on your back: like back carriers, the NIMBEL has frame bars that ensure good weight distribution to the hips. But without the distance between your child and your body. This makes it very lightweight to carry. So the NIMBEL is actually somewhere between a regular baby carrier and a back carrier with a frame.
You can carry on your back for quite a while. It can also be very handy to carry toddlers who can already walk for a short distance in the carrier.