ProTools started as a proprietary DSP system. A DSP system is a system that uses a proprietary DSP hardware and software combination instead of relying solely on the computer’s memory resources.
When computers were limited in speed, memory and space, ProTools proprietary DSP system used memory from the hardware interface thus giving the most reliability in a time when computers were not reliable. The story of ProTools popularity comes from the TDM & HD DSP based systems that offered a stable solution on a computer before computers could be stable with audio.
Now that computers are very powerful there are many other software solutions that can be used other than ProTools that do not limit the use of hardware with an exclusive software. Because of the wide variety of powerful hardware and software, it is very important to carefully consider the expense and feature list of any system before investing.