Let's start with the stats for the weapon:

Lance

Melee Weapon (Martial)
Category: Items
Damage: 1d12
Damage Type: Piercing
Item Rarity: Standard
Properties: Reach
Weight: 6
Special: You have disadvantage when you use a lance to [a]ttack a target within 5 feet of you. Also, a lance requires two hands to wield when you aren't mounted. [See PHB pp. 149-150]

Great Weapon Fighting Style cannot apply.

As we can see above, the lance does not have the two handed or versatile properties; one of which is required to benefit from the Great Weapon Fighting fighting style:

When you roll a 1 or 2 on a damage die for an attack you make with a melee weapon that you are wielding with two hands, you can reroll the die and must use the new roll, even if the new roll is a 1 or a 2. The weapon must have the two-handed or versatile property for you to gain this benefit. - PHB p.72

You could use Two-Weapon fighting instead if you are mounted and take the Dual Wielder feat, though.

Dueling can apply.

But only while you're mounted; this weapon's special property states that when you aren't, it "requires two hands to wield". Because of this, Dueling, which requires that you use the weapon in one hand, can only apply while you are mounted:

When you are wielding a melee weapon in one hand and no other weapons, you gain a +2 bonus to damage rolls with that weapon. - PHB 72

Lances are STR based.

All melee weapons in 5e are STR based unless they have the finesse property (in which case you can use either STR or DEX). While there may be ways of getting around this, a previous edit suggested you were planning a fighter or paladin, and the linked solutions wouldn't work there.

What benefits does a lance offer?

If you're thinking of playing a mounted melee character, the lance gives you reach without the two handed requirement (if you're mounted) and does more damage than any other weapon with that property (d12 is the highest damage dice available for official weapons). This means if you're playing, say, a mounted fighter, he can wield a d12 weapon in one hand with a shield or secondary weapon in the other (if you take the dual wielder feat - Lance doesn't have the light property), which is pretty impressive! So in Summary:

1. Reach - esp. on a mount, you can run around without taking Attacks of Opportunity, and you can get at monsters blocked by another character or monster.

2. d12 damage - the highest available from an official weapon.

3. A free hand (if you're mounted) meaning a shield or (if you take Dual Wielder) a second weapon. Imagine 2 lances!

No other weapon gives you all three of these at once.

However: Within 5 feet attacks are at a disadvantage, so unless you have some way of keeping your foes outside of normal melee range (5 ft), if you want the lance as your primary weapon you're going to be either missing a lot more or provoking attacks of opportunity almost every turn, which sucks a fair bit. This is the price you pay for the above advantages, though Dueling could help offset it if you don't intend to put anything in your other hand.

Opportunity attacks could be avoided by using the Mount's limited action, as the mount is the one doing the moving. (Thanks to @guildsbounty for pointing that out!)