The rule of thumb is how much natural tooth remains around the outside. Once a filling is wider than about half the biting surface, or a cusp has already broken, the remaining walls flex every time you chew and eventually one of them splits. A crown holds the whole tooth together instead of wedging it apart.
Root-treated back teeth are the clearest case. A molar that has had a root canal and no crown is markedly more likely to fracture, and a vertical root fracture is not repairable — the tooth comes out. That is why we push for a crown after endodontic treatment and why we do not charge extra to do it quickly.
The material depends on position. Lithium disilicate for anything in the smile line, because it transmits light the way enamel does. Zirconia for heavy grinders and for second molars, where strength matters more than translucency.