A monolithic chain performs every job itself. A modular stack lets a rollup handle execution, a settlement layer handle disputes and withdrawals, and a dedicated network handle data-availability.
The argument is specialisation: each layer scales independently and costs less. The counterargument is that security becomes a chain of assumptions, and your assets are only as safe as the weakest link you did not read about.
For traders the useful question is narrow: if the execution layer you are trading on stops, who can still process your withdrawal, and how long does it take? If the answer involves a committee you cannot name, size accordingly.
Related: rollup, data-availability, layer-1, sequencer