The biggest difference between modular and non-modular furniture is that the former can be further combined and adjusted, whereas the latter remains essentially the same as it was when purchased.
Modular furniture is made up of separate parts. With modular workstations, for example, the desktop, frame, screens, storage, and connectors can all be combined depending on how many people you're seating.
Need 4 workstations today? Start with 4. Team grows later? Add more desktops and modules onto the same system. And when the office layout changes, a lot of the original parts can still be reused.
Non-modular furniture is a lot simpler.
A standard standalone desk, for instance, has a fixed size and structure from the start. You set it up and use it — no combining pieces, and the price is usually easier to predict. But turning one desk into a 4- or 6-person workstation later on just isn't really an option.
Neither type is flatly "better" — it really depends on how you're using it.
If headcount is stable and the office isn't going to be rearranged much, standard furniture does the job fine.
But if the company's still growing — more hires, shifting departments, workstations getting reshuffled down the road — modular office furniture gives you a lot more room to adapt.