When audio plug-ins first arrived on modern audio recording software they helped users of the software to work inside the box. Some chose to embrace audio plug-ins and an entire industry was born.
Early plug-ins were mostly unique and generic plug-ins that shipped with the DAW, then companies like Waves, Universal Audio and SoundToys began to make plug-ins versions of classic studio hardware using complex audio modelling technology.
It's easy to get seduced by the skeuomorphic GUIs, which often look identical and sound pretty damned close. However one of the best things about modern plug-ins is that there are some audio plug-ins that do things hardware can't do.
Julian at Pro Tools Expert, the Pro Tools blog, has compiled a list of his top 5 audio plug-ins that do magic and in some cases work miracles on audio that would never be possible with hardware
Go check out his article and also the comments with other suggestions from the Pro Tools community.