... because they have forgotten to whom they have told the information.
Eventually you'd run out of things to say.
Plus, in presenting the same information to several people you can explore different points of view in their reactions and expand and explore your own realm of knowledge.
Emphasis or forgetfulness, or pretending to be forgetful to tell the hearers that ...you should be more aware of waht I say.
Because some people hear information, others need a visual word in there, some a kinesthetic word, and so forth.
I was a school teacher, and we were taught that although you may have explained something, count on 3/4 of the class as not having heard it
Probably because they find their information interesting and reuse it when it seems appropriate to do so, sometimes as a reminder. It is also something of a cultural practice.
I think I'm confused about the question. Because if you didn't repeat something you told one person, telling it to another person...well then you'd only be able to share things once. And that would get boring and lonely. And pointlessly limiting.
Or did you mean, why do people try to retell you things later, that they already told you earlier? Sometimes people forget the mentioned it, or forget who they told (so thought they told someone else and not you yet), or they are bringing it up to add something, or they think you didn't hear them the first time, or they're trying to emphasize it by repeating it because they're worried about something, or they wanted some kind of feedback from you (comment, reaction, advice, enthusiasm, sympathizing etc) so they're hoping by repeating it they'll give you another chance.
Because they're after a reaction and want you to acknowledge that you've heard and understood what they said. If they did not get a reaction they're bound to repeat themselves.