It is definitely a plus as a composer to be able to record and manage your own set of instruments. Everyone has access to the same sample libraries and often, at some point, people will use the same sounds as more and more works are created. By investing the time and knowledge into creating your own, you have the ability to make something new, and to stand out. No one else owns that sound but you.

Sound design is not just sound effects that you can pull from a library. It involves both synthesizing sounds and going out and capturing them from the real world and then manipulating them, if you so choose. It can help to be aware of the tonal centers of your music and sounds in case there are clashes from something that is pitched.

Synthesis is the concept of taking a sound generator and building upon it's basic waveform to create something more complex. With synthesis, whether an analog hardware synth or a digital plugin, one has complete control over all aspects of the end result wave. This involves pitch, envelope, ADSR and any other parameter you can think of. In my personal experience, the best way to really understand how this works is to just play with a synth of any sorts. Be aware of what you are manipulating and how that is making things change. Envelopes change the shape of a waveform while the filters change what frequencies and resonances are apparent to the listener.

In a related area, audio signal or digital signal processing is another option a composer has to make their own sounds. These can be anything from standard effects to synth like engines that one can use to alter pre existing audio data. You can think of your initial data like your waveform. As you apply effects to it and process it, you sculpt it into something more complex, just like with the synths. I enjoy putting the two concepts together when I have a project. I will generally make my own synth pads or drone sounds in Reason or Massive. If I need a more complex sound I may start with a pre-existing piece of audio or sample something and then add DSP on top of it to create something entirely different.

Back to what I mentioned earlier, your own recorded and synthesized samples are what you will end up using the most once you start collecting and establishing a bunch of sounds. You can record yourself even, or record other live players. Then you can manipulate and change the sounds to however you need to fix them. In some instances, the more wacky and messed up the sound is, the better it will fit the context you're using it for. Taking what you know from synthesis and DSP you can take and completely invent new material and then sample those back in and rework them some more. I took a bass drum hit once and added lots of reverb and saved that super massive hit sound. Later on, I took that new sample, and added delay and a pitch shifter and a new reverb and made a pitched percussive sound that did not sound like any known piece of percussion. What mattered was that it worked in the context of the project I used it for.

What it comes down to is experimenting and finding what works for you and what does not. There really is no right or wrong way to create sound design, just what works for that project. You may make the coolest sound, but if it does not fit then it's gonna sound bad. What's great, though, is you can save it and it will just live on a hard drive until it is needed.