They are there for the same reason as the rest of us, to participate at mass. Now, music has a proper place in liturgy, and as such it should be done as beautifully as possible. So it makes since that there is a group of people who are talented, take time to practise, and focus on leading the congregation’s music. But they are, from a liturgical standpoint, not like priests, deacons, lectors, altar servers, EMHC, etc, they are simply part of the congregation. So having the knights join them is now wrong from a rubric standpoint.
Now, the designs of many modern Churches get this all wrong. Choir space is laid out facing the people, for example, as opposed to being oriented with the people towards the altar. It leads to the misunderstanding of the choirs role and the idea of them putting on a “performance”. I think that is where much of the confusion originated. Back in the days where every church had a choir loft, this confusion never occurred. Now the intent of not having choir lofts was not bad, per se. The intent was to stress even more that they were part of the congregation. Unfortunately, the execution of that intent was often flawed.