Do you know the feeling of making something all by yourself, finishing it, being very proud of it? At that point, have you ever wondered whether the thing you made is really actually any good or as good as you first thought it was?
Most of the time when you write something, this es exactly how you feel. And worst of all, you know you're not being objective, you can't be, because the story you just told is your baby after all. At this point, the beta reader is invaluable. First of all, a trusted beta reader will actually agree to spend some of their time reading through what you wrote. This is not a small sacrifice for most people, and I very much respect and appreciate that. Secondly, they will tell you the truth about what they think of what you put on the page. They will take it apart. They will show you what and why things don't work. They'll be straight with you.
It is this honesty that always makes me think how much you expose yourself when you ask someone to beta read something for you. Sure, submit your writing anywhere and get rejected, keep doing it over and over, that is a certain kind of exposure as well, but the thing with a beta reader--in many cases a first reader--is that your story just came into being and is probably full of faults and rough edges that still need to bee smoothed out or sharpened to a point, so it is much more personal than submitting finished work anywhere.
The tough thing thing is that only honest criticism can show you how to move forward. The beautiful thing is that being open to such criticism will get you moving forward, for sure.
So if you're a writer, pick your beta readers well. My experience is that the people who do find what's not working and tell me make the best beta readers. Someone who goes easy on you may make you feel good, but it doesn't help you improve your writing much.
And when your writer friend asks you to read something of theirs, don't take it lightly: know that it shows a lot of trust on their part, and know that in most cases, it took them some time to actually decide to ask you! Because showing someone your words for the first time is not an easy thing.