Nobody can accurately predict a P&L five years from. But, most entrepreneur have a pretty good idea about the cost of running the business over the next 12 months. Product development, giving a free 1 year subscription to new customers, designing the marketing materials, hiring an investor presentation designer, etc.
Still there is value to putting all these floating numbers in a coherent P&L. In the short term, you can fix the exact timing of your expenses. "Free subscription to new customers" is a zero entry in the budget, but in a P&L becomes a real cost. Thinking about year 5 forces you to do the check whether your business is actually vaguely plausible. Can you recruit the required 10,000 enterprise customers with a 5 person sales force? Is it viable to have $500 server cost per customers if they are only paying $400 at best, before even getting to R&D and marketing costs?
The P&L is a thought exercise, not a tool to calculate your $20m profits in year 5.