The main uses for the Brush Guards are to protect them during travelling or washing. I haven't travelled with them just yet, but I did use them for a wash and pictures will be below :)
I purchased the Variety Pack which comes with 6 guards - 1 big one, 2 medium ones, 2 small ones and 1 tiny one, and including shipping, it cost me about £5.50. Bargain!
So here is how I used them. After I washed the brushes, I stuck on the appropriate Brush Guard, and then dried them upside down in a cup.
As you can tell, this is a major plus, in that you can dry them in a way you KNOW that the water wont get down in the ferrule and ruin your brush. Can't do it without these!
The picture on the left is right after I washed the brush and put the Brush Guard on, and the right is how it came out. The Duo-Fibre brush was a bit disappointing as it did not dry completely overnight. I'm not sure if it has to do with the ventilation of the Brush Guard, as it usually dries fine on its own. However, the other brushes I did this with (which are all smaller) dried completely fine.
The second thing I noticed was that you really need to get the size of your Brush Guard right to make sure it doesn't make your brush shape strange. I had this issue with my Sigma Large Flush Brush E50, but it was fine after I wiggled it about on the back of my hand a few times. Of course for most brushes, like my Sigma E35, the result was actually less splay-ey than usual after a wash:
The third thing I noticed was that the bristles seemed a lot softer than usual. I'm not sure if the Brush Guard had anything to do with it but they seemed REALLY soft.
So would I recommend these? Yeah, I would. Especially if you travel a lot, and also cuz they are cheap and they give you a sense of ease because your brushes wont splay. However, I'm not sure they are a necessity if you don't travel, if you already have a travel brush kit, or if you don't have pricey brushes that you are insistent on keeping for 5 to 10 years. I'm personally glad that I bought them, as I really am keen on taking care of my small collection of expensive brushes.
Also, I would recommend the Variety Pack for starters, as you really have no idea what size your brush would be suited for and whatnot. I think after this pack, if I were to get some more as time goes on, I would probably stick with this pack because there are some brushes that I have no fear of splaying (eyeliner brush, stiff shader brushes etc.,) and I don't have a massive collection of small brushes or big brushes.
Hope that helped! You can purchase these from Diane and Malcolm here.