Hello, im KELLY george from youtube.com/c/kellygeorge and here are 3 things that collaborating helps with.
View flow - when you collab with someone they usually put your link or say for there viewers to go watch the video you guys made together witch in the end help you get you many views (especially is they are bigger than you)
Following - It will help you get followers especially if the person you are creating with has a different type of video, because not every one follows the same person they mite know about and then subscribe and a lot of people will come to follow if they aren’t used to your content so many new faces will appear and they mite realize they like you better
friend ship - you could collab with someone once and be done but if you become friends with that person you mite have done something wrong or your channel is having a dry spell they will collar with you anyways just to help out instead of for views
summery - it helps you get views, followers, and mite help you get back in the lime lite if you aren’t in it anymore or were never in it