If you're like me, you like to throw out/recycle anything that is old and shoddy and presumably has no use. I have limited space, so I'd prefer not to occupy it with such items, especially when I'm likely going to replace it if it's something that I need. I certainly can't make room for a new AND old dustpan, set of throw pillows, floor lamp, etc.
But, paint brushes are a different story. Why, you ask, would I keep something like these on hand?
They aren't pretty. They've seen some rough times. But they occupy a place on my tools shelf all the same. I keep them to use with more aggressive tasks, and I get to spare my good brushes in turn.
Whatever paint medium you're working with, you'll need to mix a color at some point, no? Consider for a moment how you mix paints or inks. What's happening to your brush when you do it? They get a bit of a rough treatment, don't they? They're swirled around in a jar or brushed against a palette, over and over and OVER, sometimes with the tips of the bristles bluntly impacting a surface. Quite simply, it is not good for the brush.
In addition to mixing, brushes can suffer a fair amount of abuse while lifting paints from watercolor pans. Some pans are softer than others, sure, but any way you look at it, your brush is essentially scraping paint off of a surface. Now, of course this has to happen in order to get paint on your brushes, but if you need to pick up a fair amount of color from of a pan to mix with another color, you'll apply a bit of stress to your brush.
Opt for a semi-retired tool instead so your good brushes can do what you want them to: create your beautiful work. I keep small to medium size brushes for mixing up portions of paints (be it watercolor or oil), or stirring up my gouache that I've stored in a jar for later use, etc. I also keep smaller size brushes for loading my pen nibs with watercolor or Finetec. These brushes literally do not do anything except serve as an applicator for my nibs. I don't use the same brush for nib loading as I do for painting.
If you currently don't have any "waste" brushes, it might be worth purchasing some. You can easily find cheap brushes, sometimes for $1 or so at an art supply store. You don't need a good performance from these brushes when it comes to producing actual artwork- you just need something that can do the dirty work so that your $10/$20/$30+ brushes can survive as long as possible.