Every site is different, however as a site grows to a certain point many start looking at ways to:
1. Add advertsing in a way that's a bit more aethetic than banners inserted at the top of everything or between posts (top banners are nigh useless as banner blindness set in for most folks long ago, the CTR on them is VERY low compared to other placements.) And skyscrapers are likewise low paying and take up waaay too much space as well as irritate members. 300X250 is the current optimum, and fits into column based layouts perfectly.
2. Ways to tie other parts of a site/network into each other.
3. Places to spotlight featured events/articles, popular posts and recent projects etc.
Due to the variability of individual sites; trying to codify exactly what, when, and why is pretty much a waste as it varies so widely . What's important is having the ability to control the page layout by adding columns and blocks in those columns where they are appropriate. Specifically, it's better to be able to implement them so that the primary content loads first (for SEO) and the columns are added later then moved into position via CSS.
I would design it to be able to specify that individual blocks can be defined, specified as either right or left hand column and what order. You should be able to specify that a given block shows up either everywhere, everywhere *except* specifics sections/forums or only in specific sections/forums.
This would greatly simplify the creation of a much wider array of forums, so that not all XF implementations had to either look just like each other layoutwise, or have to be heavily modded to provide a different layout.
There may well be something in the existing system to accomodate this, but until we can get a peek at the backend, it's worth mentioning.
Essentially this would accomodate what most people use the vBadvanced's CMPS system for as well as the many side column mods for vB. Most monitors are wide enough today that having just the thread covering the entire width of the page is no longer necessary, and with many sites using a fixed width layout, columns are a somewhat obvious part of modern web design (in many cases).
I'm talking about something a lot more flexible and useful. Personally I would only have use for a right hand column as English is a left to right language and having anything to the left of the primary content (thread/posts) can be visually distracting, though a secondary nav can work there in some cases. But for right to left languages it's obviously the opposite so they would likely prefer a left hand column and nothing on the right.
but I would never pretend to know all the uses that a creative designer might come up with, that's kinda what creative means... Coming up with new and different ways to do things .