A root canal treatment weakens your tooth for two reasons. Not only does tooth structure have to be removed to accomplish the treatment, but your tooth, which is now a dead tooth, will become dried out and brittle because its blood supply has been cut off.
Back teeth are vulnerable to vertical fracture. Since they have more than one cusp, biting down on them can cause a force that tends to separate the cusps. The resulting fracture can ruin a tooth if it propagates down into the root. So molars and premolars should have dental crowns after a root canal treatment, which will totally prevent this type of fracture. And to help retain the crown, or to strengthen the neck of the tooth, a dental post will often be placed.
In times past, these posts were made of stainless steel. But stainless steel contains nickel, and many people have metal allergies, which usually means that they are allergic to nickel. In the 1980s it was discovered that some metal ions could leach through the tooth and into the bloodstream. So in the 1980s, titanium began to be used for posts because this is a very bio-compatible metal and also is very strong.
In the 1990s, other materials were introduced. Carbon fiber and other fibers were used to make posts. They were found to reduce root fracture, especially on anterior teeth, because they had a slight amount of flexibility to them. And then white translucent materials such as fiberglass began to be used because of the popularity of translucent all-porcelain crowns. Metal and carbon fiber posts prevented light transmission through the tooth and affected the aesthetics. In my practice, I began using these fiberglass posts on front teeth as soon as they came out, for two reasons. First, their slight flexibility helped prevent root fracture. On front teeth, the stresses on the teeth are mostly horizontal. Tooth structure has a slight flexibility to it, and if you take a tooth and flex it a little bit horizontally but it has a rigid post going down into the root, that could cause the root to shatter. However, if the post is flexible, it prevents all that stress from being transferred to the root.
Second, if I used a white-ish and slightly translucent post I could re-create the natural translucency of the tooth without having to mask out an opaque metal post or a black carbon fiber post.