Q - Do we really believe we won’t go to Hell if we wear a scapular and pray every day? More generally, there are things not in the bible, like the rosary, etc. that many people say are unnecessary. Why do we have extra things? Isn’t being good and following God’s Word good enough?
A - Thanks for the question. I will get to the scapular issue toward the end of this post.
While there are Catholic "extras" that aren't necessary for salvation, they can help us grow in holiness - these are not part of the deposit of revelation we call Sacred Tradition, but traditions (with a lower-case"t"). An analogy might help. You could get a hot dog with just a bun and the hot dog. But, every person likes to dress them up differently. I personally like them loaded with ketchup, mustard, onions, etc. This is the same with all the Catholic extras, such as the different forms of prayer (rosary, praise and worship music, adoration of the Blessed Sacrament, etc.) and different devotions (Marian, saints, different sacramentals, etc.). We don't all have to do the same things, because we are all different and have different spiritual tastes. Just because a devotion isn't found in the Bible doesn't mean it isn't helpful to us spiritually.
Now, back to the scapular. There are several different kinds of scapulars and each has a particular focus, but this is a particular kind of spirituality that comes from wearing a cloth pendant. The origin of the scapular is found in the religious communities and their clothing, called a habit. The cloth pendant identifies the wearer with the spirituality of a particular religious community, for instance the brown scapular is connected with our Lady of Mount Carmel and the Carmelite communities. Thus, a wearer of a scapular that is associated with a particular order is to be inducted (called invested) into a community by a priest.
Regardless of what community a scapular is associated with, each is an outward sign of their devotion to Mary. There are particular promises that are associated with some scapulars. The most common is the brown scapular which is said to free one from the fire of hell if worn at all times. This isn't a magic charm that keeps you out of hell. Rather, it is a promise that if you maintain your relationship with Christ and His mother that your are assured of heaven through His grace and your faith.