When one-off billing makes sense
For one-time projects, with clear deliverables and defined deadlines, an invoice per stage or at completion is more transparent than a fixed monthly amount that doesn't reflect genuinely variable work.
When recurring is the right choice
For maintenance, subscriptions or retainers with a steady monthly workload, recurring billing removes the repetitive work of manually generating the same invoice every month.
When you combine them
Many businesses use both: recurring for the base retainer, one-off for extra one-time projects requested by the same client. You don't have to pick a single model for everything.
Bottom line
Your billing model should reflect what the work actually looks like, not be chosen out of habit. Revisit it if the relationship with a client changes significantly.