An Onlay is a Less Aggressive Option Than a Crown
If a tooth has a large filling and is showing a sign of structural fatigue that could eventually become worse, that tooth does need structural support.
However, if the tooth has not broken yet then an onlay can be the more conservative approach. A crown will cover the entire biting surface of the tooth, as well as the tooth structure above the gum line. Compared to a crown, an onlay is a less aggressive restoration when one can be performed, since less tooth structure needs to be removed in order to place the onlay.