The spey blade is part of the game when it comes to trapper knives, but not a general rule. You may find trapper knives without this blade – they come with other types of blades instead, designed for other purposes.
Decades ago, the spey blade was mostly used to neuter livestock. With time, its uses have diversified a little. The blade became popular among fur traders and hunters. It has a curving point and a dull tip, meaning you are less likely to puncture.