If you have a form submission that updates multiple tables - say, adds you as a customer, saves your order, and adds it to a queue - then a transaction ensures that either ALL of the tables are updated or NONE of them are. In this way you don't end up with half-orders, or data that you can't use later.

It's far better user experience to fail, than to lie about having succeeded and throw the user's data away.

Transactions are one of the most useful tools in the data toolbox... you can get work done without them, sure, but most of the time you just end up creating something that looks a lot like a transaction, but slower...

2. Error handling becomes much easier if you can raise an error at any point during request processing and it simply rolls back the whole transaction, since you don't have to duplicate all validation to also run before you first write to the database. One important case is if there is a bug that triggers an assertion failure in the middle of the request processing.