In my long career in different businesses, I have been involved with two types of businesses that use large cash transactions on a routine basis. Neither of the businesses are in any way illegitimate or illegal, but cash is used for convenience. If I were to tell you the businesses and why cash is used, you would likely say, "I never thought about that, but that makes sense." Both types involve a very few people and could be easily targeted if people only knew, so no more details. These businesses operate mostly domestically, but one of the two creates significant international transactions that are usually handled by wire transfer, but not always.

So yes, there are legitimate reasons to carry large sums of cash.

Are the transactions subject to IRS reporting requirement? Probably. Do they get reported. Of course they do.