What?

Reduced Ordered BDDs are likely not as succinct as an arbitrary BDD.

The famous Hidden Weight Bit problem can be more succinctly expressed in arbitrary BDDs (changing the order and revisiting nodes), but are provably EXPSPACE in ROBDDs.

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We study ROBDDs because they uniquely reduce to a canonical form. All functions have exactly one (!!!!) ROBDD.

BDDs in general however are really arbitrary, as arbitrary as any codebase can basically get. That makes BDDs in general to difficult to study or do math upon.

Results based on the studies of arbitrary BDDs do NOT apply to the simpler, easier to understand world of ROBDDs. And vice versa.

I can now elaborate on my comment above a bit more...

Ingo Wegener's "Branching Programs and Binary Decision Diagrams" copyright year 2000, is considered the introductory work on all BDD related matters. On the front cover are two BDDs: BOTH of these implement the hidden weighted bit function (HWB-4) introduced in the first chapter, and is basically the main example used throughout the whole book.

Please locate a copy of the front cover ASAP, before continuing with my post.

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Now that you've found the book cover, look at it carefully and understand the HWB-4 function. (Just traverse it by hand, dotted lines represent "x1 == 0", while solid lines represent "x1 == 1").

Both BDDs fail to be ROBDDs: the left diagram explores the variables in this order: x1, x2, x3, x4, and then (depending on path), x1/x2/x3. That is, the variables x1/x2/x3 are explored _TWICE_ (breaking the rule for ROBDDs).

The example on the right also fails to be a ROBDD. The variable orderings are seemingly random: one branch covers x4, x1, x2, x3. The other branch is x4, x3, x1, x2. This fails the "ordered" property as the different branches cover different orderings.

BOTH are BDDs however. Both clearly implement the HWB-4 function efficiently (less than EXP-space. Indeed, its a very small graph for the front cover of a book). If these were actually ROBDDs, they'd be ridiculously large and unable to fit.

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ROBDDs are again, studied due to the unique property (and proof), that for any given function, there can only be one ROBDD ever generated (!!!!). For the verification problem of circuits, this is extremely useful. Different circuits might implement the same function, despite one circuit being smaller or using less power (or other useful properties).

To "prove" that the circuits output the same function, you simply create an ROBDD. Because functions ALWAYS result with the same ROBDD, you can ALWAYS use this process to prove circuit equivalence.

ROBDDs aren't succinct. No: their usefulness is the opposite. Exactly one ROBDD to ever find for any binary function that has ever existed or will ever exist.