Broadly, I’d say that that main benefits of a correspondence course are the systemic approach, the ability to get feedback from a mentor (for courses that offer that), and the accountability of having a specific progression.
For a true correspondence course (i.e., periodic lessons that must be completed and returned in a specific order) the systematic part is that (if it’s any good) it will cover its subject in a lot of depth, that it will break up that material into pieces that make it easier to really make progress in covering that depth, and that it will force you to spend time with everything in the course rather than letting you skip over the parts you struggle with. I think the last part is the most valuable. I’m nearing the end of the Dolmen Arch course myself, and I can say that I’ve benefited from and come to value every single practice and teaching included in the course. But honestly, if I hadn’t done it as a course, I probably would have dropped more than half of it long before I started to get the real benefits because I didn’t find it immediately congenial.
The ability to get feedback with a mentor can be very significant too. I’ve relied on guidance from my Dolmen Arch mentor (Hi Kathleen!) a lot over the years. Recently, we discussed some issues I’ve been having and were able to root out and correct a long-standing technical mistake I’ve been making which had some pretty serious practical implications. (Most Druidic practices are not really that technical, but some are.) The Dolmen Arch material is also available as a published book, so you don’t actually have to take it as a correspondence course, but this recent experience alone makes me glad I chose the mentored course route.
As far as accountability, some people find they make a lot more progress when they’ve paid money for a lesson and they have to complete it before they get the next one and there’s a real human on the other side waiting for their completed lesson.
But really, the answer varies a lot based on which correspondence course you’re talking about.
I’m not involved in the OBOD or the BDO, but as far as I know, the material in their correspondence courses is not fully available elsewhere, so if you’re interested in one of those specific approaches to Druidry, that’s the only way to go.
The ADF Dedicant Path, which I started a couple times but never finished back when I was an ADF member, isn’t really a true correspondence course, since you just get the whole book at once and finish the requirements on your own pace, though Michael Dangler’s optional companion book reformats things in a more correspondence course-like fashion. In any case, there’s not really anything in it that you can’t find in other ADF (or Ian Corrigan) publications, so the whole point here is the “dedication” part of “dedicant,” not access to a specific body of teachings. A majority of members never actually get around to finishing the DP, though it certainly has value for those who do.
The Dolmen Arch, as I mentioned above, is available either as a course with a mentor or as a published book. It’s an amazing course and it has absolutely become my foundation, but I don’t think I would have been successful working with this particular material if I had tried to do it alone from the book. Ironically enough, now that I have that foundation, I don’t really intend to do another correspondence course for a while, and I feel like I’ll be able to use material in any format much more effectively.
Outside of Druidry, I have experience with Christopher Warnock’s astrology courses. They’re excellent, but if he published the material fully in book form, I’d probably just stick to the books, honestly.
I also did the Order of Essenes’ New Thought courses when they were available only in true correspondence form. Now you can find all the lessons online. In this case, having a mentor was fun but unnecessary, but doing the lessons over a long period of time on a weekly basis is where the main benefit comes from, and just reading through them all at once wouldn’t have anywhere near the same impact. In this case, you can skip the mentor (they’re hard to find now anyway), but not the format.
The other day, I was looking at the website for a certain esoteric organization that claimed their correspondence course had the most powerful and effective spiritual teachings that ever existed in the whole world and that doing the course was the only way to access these. They went further and claimed that everything they published was actually printed in a kind of “code” such that even if you got a hold of their private instructional material from a second hand shop or something, what you read would only mislead you further. Since I chose not to take their course, I can’t say whether or not they’re telling the truth. Druid groups generally don’t make such claims, though, so I don’t think you have to worry about that sort of thing.