When you get more revenue you might decide that you want to release more products, develop faster or explore other verticals, and these efforts might require hiring more developers - that is totally justified.
However, if you are able to solve your current & near-future business problems with your current headcount there is no valid reason to increase it. Increasing it not only means more salaries need to be paid but also incurs exponential complexity in managing a larger workforce so this is also something that needs to be taken into account.
Employees can also become a liability when you no longer have enough workload for them or if there's a downturn and your revenue is no longer sufficient to pay their salaries. Layoffs are not easy, both from a financial point of view (your company might need to pay severance packages, etc) but also from a PR point of view.