When the pulp, or nerve in a tooth dies and the nerve canal inside the root is disinfected and sealed up with an internal filling (root canal filling), the tooth no longer has a blood supply. It’s a “dead” tooth. If it happens to be a tooth which gets used for chewing like the back teeth do then it takes a lot of biting force. Usually a tooth that has a root canal- type of problem has already had it’s share of previous problems and dental work. It may have had a large and/or deep filling done years ago. This by itself will compromise the strength of a tooth. The nerve/pulp inside may succumb to problems years later. In any case a root canal filling doesn’t strengthen a tooth. It only makes it’s innerds pain/problem free. However, like a dead tree branch this tooth is more prone to bigger fracture problems than a live tooth. Especially if its one of your heavy “chewers”. Covering the tooth with a crown/cap or other type of “lid” will prevent you from chipping, breaking or splitting off more of the tooth. If an internal crack develops before a crown can be placed, it typically starts out small (like a crack in your windshield). However given enough time and chewing the crack can extend and eventually a piece of tooth can break off simply because it was ready to go. A crown /cap works like a helmet for the tooth preventing future fractures. Front teeth are not as at risk of this as back teeth because we (normally) don’t chew with our front teeth.