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Transactions are at their best when complex database operations that involves many changements in different tables need to be atomised. Or all pass or no one. It prevents that updates are only partly done in eg a ledger system to avoid unbalancing, where always two accounts has to be modified, one to credit and one to debit.
I know some modules use it, but I assume that XOOPS itself doesn't need it, because it runs already for years without problems on ISAM.