While you have asked an excellent and bvery important question, I'm surprised that you didn't mention, perhaps, the most important husband, the Ba'al tefilah. So, with your permission, I'll explain why the need for this role.
Years ago, chazzanim were more common. They learned the art of nusach and singing in shul, and each Shabbat and Yom Tov, they gave a musical "performance". Yes, many of them croaked or yodeled and the services were long and ended on Shabbat about noon or so.
Have you heard most of the guys who pray, now? They have poor voices, don't know the nusach well, and basically, just sing Carlebach tunes and mumble or rush through the davening so that everyone can get to the chulent before it gets cold and hard.
So any shul that uses just any non-professional davener to lead the service needs a husband to tell the prayer leader how to sing properly, pace himself properly, etc. What's interesting is that this husband is also polygamous and is a husband to more than one prayer leader! (I won't delve into the legality of this, but, essentially, an exemption is made for religious reasons.)
Those shuls that still use a professional chazzan, don't need a husband, because these guys know what they're doing.
As a Torah reader myself, for many years, I can tell you why they call me the husband of the reader, in my shul, at least. The "reader", in this case is the gabbai. In many cases, one or both of the gabba'im incorrectly "correct" me. I.e., they tell me something I said was wrong, when, in fact, it was correct. In one case, the gabbai was looking at the wrong verse! So, as a husband, I have to politely remind him to clean his glasses, and keep on track. Granted, this situation may be somewhat unique to my shul, though. Shul politics dictate whom they choose for gabba'im in my shul, despite my occasional complaints to the rabbi.