Most amp simulators will also offer cabinet simulation  – often abbreviated to ‘cab sim’ – which makes sense given the importance of speaker cabinets within a traditional set-up; you don’t tend to get a lot of sound without a speaker after all!
Cabinet simulation is typically generated by utilizing impulse responses (or IRs), which carefully model how a real speaker reacts to different frequencies and dynamics, so that it can be replicated digitally. The benefit of having cab sim is that it gives your amp model a more authentic sound, with the nuances that a speaker provides. Many of the best amp simulators have the ability to change the cab sim too – even allowing you to upload your very own impulse responses – which is an option you couldn’t realistically have with a physical amplifier, without a significant stage crew lugging around a plethora of different speaker cabinets.