Finn as a character has no point. If I were to ask you to list three adjectives to describe him based on TLJ, could you even do it? He literally could have been replaced by anyone.
He's the first human stormtrooper. Which is to say, he is the first stormtropper we've seen who goes on to reveal his humanity. His background heavily implies that he's a child soldier and that apparently all of the storm troopers are undergoing conditioning now to force compliance. Finn has no memory of his parents, implication: the First Order is raiding worlds to staff their military machine. Finn does a great deal to grow the world and the lore of the franchise.
In theory, Finn actually has a lot potential here. There are several opportunities for the films to use some of this set up; they just don't. How does Finn reconcile his current actions with the fact that he's killing his former brothers and arms, and the only other relationships he's ever had? Does Finn appreciate the moral quandaries that come with killing brainwashed child soldiers vs, letting the First Order continue it's galactic conquest? You'd think he'd feel for the child slaves on Canto Bight given his own history wouldn't you? Does he have an impression of the future beyond the immediate week or so? What actually is the nature of his relation with Rey or Finn?
Sure, some of these are too heavy for the silver screen, yes. But the character has depths that need to be explored, or else the script flounders looking for something to do with him.
This is exactly my entire problem with Finn. He has so much potential to have such a great and interesting backstory that contributes a lot to his actions, which could have huge impacts on the entire rest of the galaxy. Instead he's just a one-liner machine so far. Probably the most egregious case of Disney Disneying up a character in the new Star Wars stuff. ---
You're such a boring characterless entity. Try getting laid once in a while and maybe you'll have friends and find out what a "joke" is. - derrate