First, let's take a look at the Spear (D&D 5e Player’s Handbook p. 149):
Spear (Simple Melee Weapon)
Cost: 1 gp
Weight: 3lbs
Damage: 1d6 piercing
Properties: Thrown (range 20/60), Versatile (1d8)
I ask this because, when you check the Trident (D&D 5e Player’s Handbook p. 149)...
Trident (Martial Melee Weapon)
Cost: 5 gp
Weight: 4lbs
Damage: 1d6 piercing
Properties: Thrown (range 20/60), Versatile (1d8)
It does the same thing, but it's more expensive and heavier.
So... Is there any reason for you to use a Trident instead of a Spear? I mean mechanically speaking, of course. Because everybody knows that tridents are way cooler.