Prepositions – What’s the Meaning of “Be Good of Somebody”?

If for = in support of seems "weird" in this context, try thinking of it as heading for, pointing in the direction of, as in "I'm for bed". But I think it's just as "intuitive" to see OP's example as short for...

The weather forecast is for it (the weather) to be sun/sunny.

Note that there's no real need for a preposition at all in OP's exact context, and semantically it's irrelevant whether it's sun or sunny (both are perfectly valid and commonly used).

When something simply collapses, it falls more or less straight down. While it was standing, it occupied a certain amount of space on the ground. When the collapse is complete, the thing is much shorter, but takes up more space on the ground, because it spread out or tilted over as it fell.

If you picture a person standing, they occupy only about a square foot of ground space; when they collapse, their knees would buckle and the start of the fall would be straight downward, but not everything bends in the same way and the person would end up lying on the ground, probably taking 5 or 6 square feet of space.

When something collapses in on itself, it falls more or less straight down, and it ends up taking no more ground space when fallen than it did while standing. Obviously a person can't really do this, but a building can be made to do it. (check out wikipedia's page on Building Implosions for some images and videos.)

Both prepositions are necessary; you can't collapse on yourself because the on, when used alone with collapse, is describing where you land. (You might "collapse on the floor", or "collapse on the bed"; but "collapse on yourself" sounds like you end up floating in mid-air since you didn't actually land on anything else.) And you can't collapse in yourself because in, when used alone with collapse, describes either the general manner in which you end up, or something that you do while collapsing. (You might "collapse in a heap", or "collapse in a flood of tears".) When you combine both prepositions, though, in describes the direction of your collapse (as being not a normal "spreading-out-as-you-fall" collapse) and on now specifies correctly where you (or the parts of you that did not spread out as normally expected) landed.