Charisma increases your speech and barter score, and if your charisma is too low some interactions will not be available , regardless of your speech or barter skills.
According to the wiki though, these interactions ultimately do not have any significant effect on game play, nor do they provide any special bonuses.
High charisma is also required to obtain the Ferocious Loyalty and Animal Friend perks.
Ferocious Loyalty grants your companions a boost to damage resistance when you drop below 50% health, and Animal Friend will prevent animals from attacking you unless you attack them first.
Something to note about Animal Friend however is this: Deathclaws are not classified as animals for the purposes of this perk.
Neither are any variety of arthropods such as radscorpions.
One final thing to mention is that it also increases companion nerve.
Description
according to the fallout wiki: Companion Nerve gives each companion in the party +5% to their damage and armor, up to a maximum of +50% at 10 Charisma.
gaming.stackexchange.com/users/8350/Wipqozn
Charisma is only useful for 1 purpose in Fallout NV.
Companions.
Like fallout 3, charisma really doesn't affect anything since most checks are based on your barter or speach skill level, not your charisma.
1 charisma and 100 speech is the exact same as 10 charisma and 100 speech.
Its better to max intelligence for more skill points to get your skills up to high levels.
Now, again the one thing that charisma does affect in NV that it didn't in fallout 3 is your companions armor/attack bonus, which is up to 50% at 10 charisma.
This is a huge amount, making tough companions even tougher.
This is usually only good if your making a non-combat character and are using your companions as the main meat in fighting.
A 50% boosted companion can kick major butt even in very hard mode.
So, 1 charisma is good for most games, but if your playing a non-combat character for roleplaying, etc max your charisma and intelligence for easier play.
gaming.stackexchange.com/users/64982/jim
Charisma i've found is actually very useful in Fallout New Vegas
I highlight that because charisma is more useful (more specifically Speech) in New Vegas than in its predecessor Fallout 3.
The greatest advantage of a high charisma is the high speech skill that comes with it which can give your character more options/choices in-game e.g. convince someone to give you a better reward for killing someone etc.
The other great advantage is that the high barter skill is great for selling items to vendors because it actually increases the amount vendors pay for your items and reduces the amount you pay for vendors items.
Overall when I finished New Vegas I actually regretted not investing more into my Charisma (Speech) because I lost out on lots of opportunities in the game making Charisma very useful and a major priority if you're playing the game again with a different character.
gaming.stackexchange.com/users/11636/OliverBM
As Wipqozn says, Charisma doesn't change the gameplay much, except to allow a couple of perks and to make your companions more powerful.
(You can have two companions at a time: one human companion and one non-human companion (i.e. a dog or robot).)
If you want to play the diplomat, you want the Speech skill, not the Charisma stat.
Charisma gives you a (small) boost to your initial Speech skill, but otherwise is not nearly as useful as Speech.
gaming.stackexchange.com/users/9522/PotatoEngineer
Being the person that made those wiki edits to add the bonus companion damage I can tell you that 50% companion damage on a companion like Cass (with her quest damage perk also) is huge.
Give her a strong gun, a unique even, and she puts Boone's famous kill stealing reputation to shame.
Light machine guns are a good choice for her.
Her quest perk gives her another 50% damage boost.
gaming.stackexchange.com/users/52806/Mictlantecuhtli
I'm beginning playing this game and I'd like to know if Charisma is worth putting a few points into.  I always play RPGs as a diplomatic character, and I want to do the same here. However, it seems that in F:NV only the Speech skill is really useful when it comes to finding a diplomatic way to solve quests.  Is Charisma worthless, even for a diplomatic character? Are there many quests where a Charisma check is made instead of a Speech check for example?
Is Charisma a useful stat in Fallout: New Vegas?
gaming.stackexchange.com
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gaming.stackexchange.com/questions/27768
gaming.stackexchange.com/users/1667/Emiliano
Charisma increases your speech and barter score. It is very useful in Fallout New Vegas but it doesn't change gameplay much. The usefulness of Charisma lies in the empowerment of companions and the enhancement of some skills.
The main utilities of Charisma seem to be for the empowerment of companions and the enhancement of some skills.