But these tools save you time for a tiny amount of money. You don't need something expensive, any startup should be able to afford $10/mo for Xero ( https://www.xero.com/us/pricing-plans/ ).
As your startup grows, you’ll engage an accountant and hire a bookkeeper or business manager, who will expect to work in the traditional accounting cycle. You’ll also need to produce the usual reports and financial statements for your bankers, investors, or a potential acquirer’s due diligence team.
Using software designed for the purpose makes all this easier and more reliable. It also conveys that your company is serious about its business: investors want to take a risk on your technology, market, and execution, not on whether your homebrew accounting system can generate an accurate cash flow statement.
It also means you waste future-you's time too. For example when you want to integrate apps into your books (sheets is not reliable at scale), when you want to train a junior accountant to look after your books. When you want have to send a copy of financial statements to banks/tax-offices/investors whatever and they are used to a particular format.