It seems that whenever "if and when" could be used, just saying "if" would work fine. Is there any situation where "if and when" is different than just "if"?
If Alone does not require the consequence to happen right away:
If the rooster crows, get up. (Does not specify that you must get up at that time, just that you must get up at some point.)
If you make coffee, bring me some. (Does not require you to bring me some right when you make it.)
When Alone does not imply that we talk about a consequence:
When the rooster crows, get up. (May imply that you should get up at the time the rooster usually crows, even if it doesn’t this time.)
When you make coffee, bring me some. (May be taken as a general directive without implication that you will be making coffee at any particular point in time.)
If and when establishes a consequence and requires it happen right away:
If and when the rooster crows, get up. (There is some doubt as to whether the rooster will crow, but get up at the occasion, provided it occurs.)
If and when you make coffee, bring me some. (Your making coffee is not a given, but should you do so, bring me some at the time when you do make it.)
Examples taken from the article "if and when" by James Harbeck. Emphasis mine.
Beyond the logical aspect, "if and when" has additional emphasis than plain "if", as such it is often used with a connotations of doubt - "If and when you finally decide to do it..." - even when finishing is certain but time isn't - "If and when this is done here...".