There's various reasons.
1)Some people carry a knife for self defense. I'm not gonna argue about whether or not that's a wise decision, or if knives are the best weapon for self defense, or any of that mess, but I will say, if you're being threatened/attacked and you do feel the need to draw a blade, I imagine whipping out a Cold Steel XL Voyager or Espada is more likely to scare the threat away(even if you never have to use the blade) than pulling your Delica or Super Tinker. Sure, you could say "why not pull a Ka-Bar" or "why not whip out your Busse Battle Mistress" or whatever if you want a knife for defense, but some places don't allow carry of fixed blades, but do allow carry of folders. I don't carry knives for defense, but my work is a prime example. I live in Texas, so I can carry up to a 5.5 inch blade, which means, if I did want to carry a big knife for defense, I could keep an XL Voyager or Recon 1 in my pocket, and be breaking no laws. And if I was walking down the street, I could wear an Ontario Pilot Survival Knife on my belt, if I wanted, also no problem. But my work itself forbids open carry of "weapons, including but not limited to firearms, knives, and pepper spray", so if I wanted to bring a knife for protection I work, the fixed blade is out, but I could still have that Xl Voyager in my pocket.
2)Some people can actually make use of the utility a larger blade provides. Say you're out hiking somewhere locally, nothing heavy, maybe a local trail near your house. Now, you might not expect to need a machete, so you may well not want to strap a machete on your hip, or, depending on the locale, you might not be ABLE to strap a machete on your hip even if you did want to. But if you do happen to run into some unexpectedly heavy brush, having that Recon 1 XL or that Rajah 2 in your pocket may come in handy. I know at least one person on these forums has mentioned using a Rajah 2 in those very conditions, and I recall someone else telling a similar story involving the Large Espada. Other people like the robustness of the bigger knives for their jobs, like on construction sites or whatever, places they may need to use their knife to cut something bigger/heavier than the box their latest Amazon order shipped in or the bag of chips in their lunch.
3)Some people just like big blades for their aesthetics, or coolness factor, or whatever, and don't require any other reason.
Both have seen a ton of use too. 
(Spyderco Military up top for reference...) 
