Those paint brushes sacrificed themselves so that our models could be what they are today.
Not all paint brushes are completely dead. The handles may be bad, but the bristles can be “revived” so to speak with the following. There’s plenty of Youtube vids on using The Masters brush cleaner if you look hard enough. It can be bought on Amazon or at Hobby Lobby.
They’re still good for weathering or dry brushing. You can also use the handles to hold 3D parts when painting (insert in the drain holes). 
This stuff is magic. Everyone who owns a paint brush for any use should have this.
I never knew this existed until I saw it in an aisle at Hobby Lobby today. The Masters Hand Soap for cleaning your hands of paint, oils, grease, etc.
I never throw a brush away. There’s always a use for them. If the bristles are beyond saving, I cut them off and use the handle as a stir stick. I use a specific cleaner for each paint. Tamiya, Windex. Testors, mineral spirits. Vallejo, Vallejo thinner. For those brushes where the bristles are somewhat hard, I pull out the big guns, lacquer thinner. You’d be surprised the brushes you could resurrect using it.
After cleaning, the brush will go in a stand up caddy.
Brushes that are too beat up for their original purposes can be cut off short and used for applying pigments, dry brushing, or mud/texture weathering products.