The main reason for a crown is the loss of tooth structure. Tooth structure can be lost in many ways here are a few
Decay - Cavities or Caries
Trauma - Chipping or fractures,
Tooth Wear - Grinding or habits like nail biting.
Erosion - over-exposure to anything acidic such as soda, lemons, or acid reflux.
Abrasion/Abfraction - flexural and compressive forces cause weakening of the dentin at the gum level. Tooth brush abrasion- brushing too hard or with abrasive bristles that cause damage.
Root Canal - the vast majority of the cases where this procedure is done there is already loss of tooth structure due to a large filling or a large cavity. In addition a hole down the middle of the tooth is needed to clean out the root canals. this results in a weak tooth that is at risk for breaking in the future.
As a rule of thumb I recommend a crown for 3 reasons
50% or more of the tooth is gone after all the filling and decay is taken out
there are obvious signs of fracturing or cracking
After a root canal.
When done correctly and if well taken care of (brushing and flossing). A crown can last upwards of 30 years. It can still get a cavity where the tooth meets the crown so proper hygiene is important.