Direct debits are coming very soon. You are right that at this particular moment, it will not include all expenditure for budgeting.
For the time being, the Track budgeting functionality can be used to monitor a specific budget based on the particular money you move in and spend, rather than your entire income.
As a second account, it could be used for day-to-day spending while keeping salary and direct debits elsewhere. Those are more fixed items in any case, so this should help to focus on more discretionary spending, and therefore opportunities to save and invest.
As we add the full planned functionality, including an eventual banking licence, it will be easier to use this for all spending of course.