Freelancers expense a dozen subscriptions — proposals, contracts, invoices, time tracking — against irregular income. Owning the tools outright removes a whole line of fixed costs.
The tool this job usually means renting is ChartMogul / Baremetrics, at $50-100+/mo — roughly $600 a year, every year. Clientdesk does the job for a one-time $39 — running on infrastructure you control.