Forecasting performance is an essential tool in running your business. It forces you to think through the impact of your current plan (while you can still change them!) and gives you an easy way to see whether you ended up performing like you thought you would (so that you ensure you keep learning what works and what needs to change).
When you are just starting out, the most important forecast is your cash-out date, which you find by dividing the amount cash you have in the bank, adding or subtracting any major one/offs you know about (e.g. major third-party payments, R&D tax credits) and then dividing the remainder with your projected monthly burn. So if you have £200k in the bank, and if you expect to get a further £50k in R&D tax credits in the next few months, then your starting point is a £250k balance. If your monthly burn is £50k, that gives you 5 months before your drop-dead time (and if you haven’t already started, it is probably time to get started on your next funding round!)
As you progress, important things to forecast will be around sales and revenue. If you are building a B2B company, you should obsess over them.
Make your forecast like a waterfall. That helps keep everyone calibrated as the prospects of the business evolve. The waterfall model is described more here: https://vcwaves.com/2011/01/18/the-single-best-financial-reporting-tool-ever/
As you evolve, your forecast will become more sophisticated. I have written more about forecasts and general financial reporting here: KPIs and Financial Reporting for Startups