You have to bulk when your goal is to add muscle to your frame and you lack sufficient fat stores to recomp. It’s that simple.
The populations with the greatest capacity to do a body recomposition (simultaneously gaining muscle and losing fat) are new lifters and obese individuals. New lifters can do this because their capacity to add muscle is so high, and obese individuals can do this because they have an excessive amount of fat stores. Naturally it’s the opposite for experienced lifters and lean individuals respectively.
Here’s how it works. Muscle actually weighs something and fat does too. With regards to long term changes in body weight, adults (not elderly) can reliably expect these changes to only come from muscle, fat, or both - nothing else. Short term changes in body weight will generally come from water and food and has the potential to drastically vary from day to day. Ignoring short term changes then, if an individual wants to add muscle to their frame, they need to gain weight in muscle - BUT if they also lose weight in fat at the same time the scale weight might not have changed much. As a reasonable example, say that over the course of a month a 170 lb man at 20% bodyfat gains 1 lb of muscle and loses 1 lb of fat - he will still weigh 170 lbs, but he gained weight in one source and lost it in another and his bf% is now 19.4.
When you are too lean however, losing a pound of fat is a much bigger deal. The number I’m about to give may be somewhat arbitrary, but it’s close enough. When a man is 10% bf or less, fat loss is much slower than it is at 15% and higher. It would be unreasonable to expect body recomposition to occur when fat loss is so much harder. Building muscle is a slow process in the best of times and a calorie deficit is going to slow, stop, or reverse adding muscle mass to the body. Therefore while it is theoretically possible to still do a recomp in such a scenario while maintaining overall body weight, monthly progress would be significantly impaired and small.
Bulking and cutting can be done at any time, but the optimality of that timing depends on an individual’s goals. If an individual prefers to be lean, cutting to a pretty lean state and then bulking without completely losing sight of abs might be the preferred route (staying between 8-16% bf). If an individual wants to simply look big without regard to specific leanness but also wants to be reasonably healthy they might bulk and cut between certain predermined weight ranges (staying between 14-25% bf). Other variations on goals might also exist, but I would expect most people who are bulking and cutting successfully to be doing something similar to one of these two.
When do you have to bulk? Outside of the aforementioned scenario that I opened with, it’s never not an option. What I mean is that it’s up to you and what you want to do. A calorie surplus is always going to be the most optimal environment for muscle growth, but it can occur with maintenance calories or a calorie deficit too (in a diminished capacity). Get into a desired range of leanness and bulk and cut within that range - that would be my advice.