For one example, imagine an Amazon order. You order 3 things, partly paid for by CC card rewards, maybe a gift card, and a CC charge for the balance. With transaction splits, it's easy to put each thing you bought into the appropriate expense account with the correct cost, reconcile the CC charge correctly when your statement comes and you import the OFX/QFX data, and track your gift card balance. In GnuCash, that's all one transaction, with many splits. For expensive durable purchases, I'll sometimes also go and add the serial number to the text of the transaction split from when I purchased it, for lightweight home inventory without a separate program or redundant data entry about what/date/cost.
Compare to a simpler personal finance program that just imports the CC charge and lets you tag it as a single expense category (or takes whatever the CC company says the category is), and if you ever need any more information, you're stuck with keeping a separate program or spreadsheet up-to-date.
(You could use GnuCash the same way as the simpler personal finance program, but GnuCash has more powerful features that are there when you want them.)