Investors will ask for financial projections.
In the very early stages (angel/seed, and possibly series-A), it's less about accurate projection and holding yourself accountable, but more about understanding your rationale, thought process, key assumptions and levers. Also if the model is reasonable, it can show you how big you can get and how fast you can get there.
Below is a simple version of creating a financial model for the very early stage companies.
One way to tell a wise investor from a not-so-wise one is to see how fixated they are in the 3+ year financial projections in your early days. If they are asking questions around key assumptions, levers, and sanity checks, that's good, but if they pressure you into committing into the long-term projections or be very detailed in the long-term planning, they probably have not invested a lot in early-stage startups.
Of course, this becomes very different when you go into growth stage.. hence..
In the growth stages (series-B and after), it is now really about the actual planning, budgeting, tracking plan vs actuals. Once your company reaches product/market fit and have entered the growth/scaling phase, you will need to find a repeatable and scalable model of growing your business. This is where modeling has greater utility function and missing a quarter or two is taken a lot more seriously.
At this point, financial projections will be based a lot on actual/historical data, with multiple tabs that spans budget around headcount, marketing & sales, facilities, etc. For example, sales leader work with sales ops to build prediction models, marketing leader work with marketing ops to build a flywheel that works repeatedly, and finance leader work with people leader and operations to build a budget and headcount plans that reliabily controls the expenses at scale and it has checks and balances with the overall health of the company from the business metrics perspective.