Some of the plugins on this list offer you a full suite whereas others only do a particular thing. Unlike mixing, mastering generally uses a set list of processing to achieve its goals, and most all-in-one suites will include at least one of the following:
Compressors/Expanders - adjust the dynamics of your mix.
EQ - shape the sound of the mix, for example by boosting the low end, or cutting sharp frequencies out of the highs.
Harmonic Exciters - also referred to as ‘saturation’, add extra harmonics that can increase the depth of sound.
Limiters/Maximizers - prevent your mix from clipping when it is pushed too hard, and can also be used to push the mix harder for extra loudness when needed.
There are other mastering tools like spectral analysers and metering plugins that enable engineers to get a visual on what the mix is doing, but most mastering engineers in receipt of a good mix will use compressors, EQ, and a limiter to get the final master over the line.
It’s all up to you whether you want to buy separate plugins to do each particular job or something that does everything all at once. If you know your stuff, then you’ll probably be better served by purchasing separate plugins. If you’re just getting started, we’d highly recommend picking out a complete mastering suite to help you get an overview of what mastering entails.