Sometimes you initially expect several transactions from/to the same payee. For example, you expect the monthly rent payment from a tenant, (which may be you have entered as a scheduled transaction) but in month M he/she also has to pay for some yearly tax (which, again, may exist as a separate scheduled transaction). But he/she decides to make a single bank transfer for the total. In such cases, it would be very convenient to have the possibility to select the two transactions and merge them into a single one so that the list of transactions in the bank account remains consistent with the bank statements.

Sometimes the opposite can also be necessary. For similar reasons you may want to "unsplit" a split transaction into independent transactions.