Ok. So there are many reasons why a stroller could be exchanged for a carrier, but it would depend on why you are using a stroller. Personally, I only used one when clothes shopping, as it is very difficult to try on shirts (specifically) with a baby on you.
Keep in mind that babies are 'sat up' in strollers as early as 3 months old, but do not really start crawling until 6-10 months (there are exceptions to this). So the ability to sit and be mobile are not interchangeable for many.
Generally, when a baby is in a stroller, they are not really getting freedom to move like you think. Also, the perspective at that level is not very.... entertaining. Think about it. If you have a baby ON you, they are at about your chest through eye level. You can carry them on your front, facing you, or facing out (Front Facing Out, or FFO). You can also carry them on your hip, in a hip carry. OR you can carry them on your back in a back carry, even better if in a high back carry, so they can see over your shoulder. Not only does this allow them to interact with the world at a level that most of our society interacts at, but it keeps them distracted and soothed for longer periods of time than if in a stroller. Also, the desire to be more interactive is typically developed around 3 months, and this is the age when a hip carry or a high back carry is introduced (depending on the baby's muscle tone, head control, the carrier, and the parent's skill/experience at babywearing). This period of curiosity can sometimes cause over stimulation, which also is decreased with babywearing, as they can 'burrow' into mom/dad, whereas in a stroller, there is no real way to turn off stimulation (similar with a FFO carry, which is one reason to limit that position to 20 minutes or the child's tolerance--much more complicated answer).
Add into this all of the original benefits of babywearing, and there are that many more reasons to do this.
The biggest for ME, is that my three year old (just had a birthday on the 19th), broke his stroller, as well as his wagon, and they did not 'hold him'. He could escape, run off, and get into trouble as active, curious preschoolers are capable of doing. By wearing him either in a wrap (long rectangular piece of woven, not knitted, fabric) or a soft structured carrier/SSC, he has the ability to talk with me, look around, point to the things he wants, and experiencing more without getting into trouble, hurt, or worse. Can I also mention that if you are grocery shopping, as you probably have experienced, when kids are in a cart, you have germs to worry about. Shoplifting (as they steal stuff and put it in the cart or 'car'. Throwing fits. Throwing your food out of the cart. And more. But when babywearing, they aren't touching the germ-infested areas of the store. They can't reach your purchases. They can't REALLY hide much to 'steal'. And the fits are generally in the beginning when you put them up, but they are happier in the long run instead.
Plus, if you have other children, it makes it easier for you to run after them. Every try to run with a conventional stroller? It is a bad idea. The wheels and/or carriage can swerve, flip, or fall. It is frustrating. Many jogging strollers are bulky, or don't have a steering front wheel. Etc, etc, etc.
Add into this a child with sensory issues. A child who cannot tolerate clothing, or changes in routine can make it difficult for the parents to leave the house. As one mother explained, learning to wrap her SPD/ASD daughter enabled them to leave the house with both minimal clothing (as the child was covered by the wrap), as well as giving the girl an 'escape' even in a public place (by ducking inside the carrier). Because they are on their mother's backs, eye contact isn't as required, and strangers do not tend to TOUCH your children when they are harnessed to you. On the other side of the sensory spectrum, a child who requires constant movement even as an infant may scream when put down to rest, but in a FFO carry/carrier may be happy and soothed. Another mother spent the first year of her son's life wearing him for 9-12 hours a day. Once he started crawling/walking/running, he was worn less and less as his own ambulation filled that need for movement.
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Aspie Quiz: 148 ND/50 NT
AQ: 41 (AQ-10: 9) EQ: 17 SQ: 31 FQ: 44 RAADS-R: 178
ASD Diagnosed 4/22/2016