The Shulchan Aruch, OC 606:4 says you are allowed to immerse in a mikvah on erev Yom Kippur any time you want as long as you immerse before sunset and that you shouldn't make a bracha on that immersion. Being given permission to immerse implies that I was afraid it was forbidden; being told not to make a bracha on said immersion implies that I thought that I should make a bracha. Now we have four questions:
- Why would I think it is forbidden to immerse?
- That being the case; why am I allowed to?
- Why would I think I should make a bracha on that immersion?
- That being the case; why don't I?
The first two questions arise because Shulchan Aruch is starting in the middle of a sentence, as the Mishna Brura explains. The Shulchan Aruch should have started by mentioning that there is a minhag for everyone -- even unmarried young men and women -- to immerse erev Yom Kippur. So the answer to question (1) is that usually unmarried men and women do not immerse now a days because we don't have the Beis HaMikdash, which means we don't daven in a place where one must have immersed before entering. That being the case, going to the mikvah today doesn't allow an unmarried youth anything that would have been otherwise permitted. Besides that, it lowers the level of issur for an unmarried couple to have relations from d'oraisa to d'rabanan; that being the case, better not to immerse. So why does the Shulchan Aruch permit it? Because it is erev Yom Kippur -- people want to do whatever thay can to feel more holy and it is not a day when most people are thinking about how to be sneaky with HaShem's rules of conduct. So the Shulchan Aruch say, "go ahead, but be careful to immerse well before sunset.
The answer to question (3) is that it the holiness of the day practically demands extra purity and going to the mikvah erev Yom Kippur has become such a well established minhag/custom that to not go is seen as separating oneself from community practice. Question (4) is now also answered, because it is only a minhag. (Mishna Brura there, sk 19)
What a minute, says I (here comes my value added): saying Hallel on Rosh Chodesh is also only a minhag, but we say a bracha. Lighting candles in shul on Chanuka is only a minhag, but we make a bracha. On the other hand, taking aravos on Hoshana Raba is a minhag and we do not make a bracha. What gives?
Here's my theory, for what it's worth. The Rosh says that even with the Beis HaMikdash, there is only an obligation to immerse on erev Yom Kippur when we have the ashes of the red heifer so we can become completely tahor -- even from tumas meis. However, says the Rosh, if you can't become tahor from all forms of tuma, then there is no obligation to become tahor from any forms of tuma. That, in turns, means that our immersion on erev Yom Kippur now a days is only a memorial to the immersion that is actually obligated when we have the ashes of the red heifer. On the other hand, Hallel on Rosh Chodesh and lighting Chanuka candles in the shul are not memorials to anything -- they are what they are; a beautiful and well established custom.
In conclusion, therefore, we see that a minhag that is what it is, gets a bracha. A minhag that is a memorial to a practice that we cannot do now but hope to soon (may the Beis HaMikdash be rebuilt soon and in our lifetime) does not get a bracha. That's my story and I'm sticking to it. Until proven wrong, of course.