I really don't know and I am freaking out. I usually don't use it because I believe that the verb contains the subject (STROVE method). When I started the course I used to write all the pronouns and of course, my answer was incorrect. It would be nice if someone could give us a hint :)
Most of the time, the pronoun is optional and just used for emphasis. There are a huge number of languages like this. I see that you're at level ten in Portuguese and Portuguese has this feature too. You can say Falamos português (Tunasema kireno) without the nos but if you're emphasising or contrasting, you'll probably use it: Eles falam inglês mais nos falamos português. (Wao wanasema kiingereza lakini sisi tunasema kireno.)
(Hopefully I haven't made mistakes in either language there.)
There are times in Swahili where the verb doesn't have a subject prefix though, such as with the copula ni or the habitual aspect prefix hu-, which I guess will come up much later in this course, and in those cases, the pronoun is the only way you can make it clear who the subject is.
STROVE is a way to remember how to conjugate a verb in Swahili. It's an acronym for Subject - Tense - Relative - Object - Verb stem - End of verb.
An example: niliyekusomea -> ni-li-ye-ku-som-ea = I, who read for you
Here ni- is the 1st person singular subject marker, -li- for past tense, -ye- for showing that this is the verb in a relative clause, -ku- is the 2nd person singular object marker, -som- is the root of the verb kusoma - to read, and -ea is what's called the prepositional verb ending, which is used to show that this is something done on behalf of or for someone.
Grammatically, it's plural vs singular. In usage, however, 'chakula' is just 'food' while 'vyakula' is used when you talk about foodstuff in general or groceries (at least the sort of groceries related to making food), or when you talk about different sorts of food.
By a quick google search on the word 'vyakula' one of the first sentences I got was from the title of a clickbait video on youtube: "Ukitaka kunenepa kula vyakula hivi!" - "If you want to gain weight, eat these 'foods'".
(edit: the sentence meant to gain weight not loose weight as I first wrote. I translated it wrongly because of my cultural expectations.)