Happened to me loads as a child, as where I grew up, there wasn't much concept of extension work (you did the work for your year, and passed the year or not).
I was quite far ahead, and one of my earliest memories of school is other children coming up to me and asking me to read things to them. In maths I was often sent to practice number bonds or times tables, or to correct work. I read out spelling lists. None of it was things that I was at all unsure about myself, and didn't need to consolidate or understand it better by explaining it to someone else.
I think it taught me patience, and to see how others might struggle with some things, and I enjoyed it on the whole, as I was a very adult-pleasing sort of child.
Socially, it was a bit of a disaster, as I didn't have normal relationships with peers as a result. I was seen as a bit of an 'older sister' - well enough liked, but on a different level than others in the class. I was treated as being a bit apart - someone that they would go to for help, advice, praise, etc, rather than someone who would have an equal and reciprocal friendship, somehow. I didn't really learn how to have that sort of friendship as a result, and it affected me throughout my school days.
Beyond middle school, when we were in different classes for different subjects, and this wasn't something that happened in the same way any more, I still enjoyed it and took part in an organised peer tutoring program; I also enjoyed working with younger children. I very much took on board the idea that that was my role. By high school, I'd sort of fallen into a stereotypical caretaking/teaching viewpoint, and assumed that is what I should want to do with my life. Sometimes I wish I had been encouraged to extend my own academic skills more at an early age, before I had lost confidence/learned to assume other roles. I also wish that I had had more chance to just be an ordinary child in the primary classroom, without the others seeing me as somehow different/older/other.
Mostly I preferred it when I was just allowed to read if I finished early. Or got to do filing or cutting out displays.
So I think it's a mixed blessing - can be some positives from it, when it's done well and it's clear that it is benefiting both parties, but there are also some notable disadvantages that are frequently overlooked because they are so subtle.