While ProTools® and similar DAWs are great for tracking and mixing, they are not
designed for mastering.

Mastering gear typically processes audio at 32 bit resolution or higher. This leaves
the mathematical rounding errors, that occur with any digital processing, outside the
audible bit depth. This means that mastering gear can perform processing that
maintains your mix’s bit depth with inaudible consequences.
About half of the Compact Disc mastering process has to do with preparing a
“master” that is compliant with the Redbook CD specification or DDP ( Disc
Description Protocol ) protocol. Protools® has no implementation for these
requirements.
It is a good idea to have someone besides the mix engineer master a track. They
can approach the track without bias. It is best to have an experienced mastering
engineer be that person. Mastering is quite different than mixing and requires
experience and a different mindset.