Most popular DAW software is sold with some included effect plugins. These are the plugins that are given to you as part of the DAW purchase and usually given to you as a free download link or serial number to activate the plugin. I will talk about the benefits of using the plugins that come with DAW software over purchasing extra plugins for your home recording studio DAW.
When you purchase your DAW software you WILL find it comes with many effect plugins. These plugins include reverberation, delay, compression and other plugins to manipulate and improve your sound.
Most DAW software will come with the essentials such as delay, EQ, gate, reverb and other effects. These are usually enough to get you going and record good professional sounding music and mix your tracks with a high degree of professionalism.
The first thing to keep in mind is what plugins you really need. If you're a musician who does a lot of singing or recording acoustic guitars or other acoustic instruments using a microphone you really need a good reverb plugin and maybe compression for your vocals and other acoustic tracks as well as a few other sound manipulation effects such as EQ to help fine tune your sound. If you are a keyboard player such as a piano player or you want to make beats you need effects to do that too and in most cases the supplied stock VST plugins should be enough to make good music.
I always recommend using the stock effects because it can save of hard disk space and can be easier when it comes to software upgrades and reinstalling them if you need to at any point in time.
There are cases where you need a sound that emulates a condition that you can not achieve using the effect plugins supplied with your DAW software. An example of this is when you want to emulate an analogue effect system using a modern digital audio workstation environment and you have to purchase a plugin specifically to achieve this goal.
There are many manufacturers producing VST plugins that do such things such as waves which need to be purchased separately to your DAW software. There are other sound manipulation effects that you may decide to use on your tracks that are not supplied with your preferred DAW software to get the desired tone you need which is when I recommend you purchase a plugin that meets your needs.