This type of brush is considered as a multi task brush since it's tapered. You could use the tip to easily get into detail like blending some under eye concealer and whatnot as for me, I love using this right now for blending out products on my face whenever I contour. It makes the seamless type of finish, like a natural, dewy skin even with an intense contour. Just this one brush for both colours is perfectly fine. Blending out the highlights first then the darker shade afterwards
I got this brush in the Jaclyn Hill sigma brush set. She uses it for undereye concealer, and that's what I use it for as well. It's fantastic. Instead of spending time dabbing in the concealer with my fingers, I just swipe some under my eyes and blend with this brush. I am able to control the placement and it blends my concealer quickly. That's good for me, because I usually layer a corrector and concealer under my eyes. The brush picks up excess product if you use too much, and just blends a thin layer of the product evenly. So you'll want to use a pretty pigmented concealer with this. Overall I'm very happy with this brush. I think it is fairly unique and now holds a specific important job in my brush collection.
I bought this brush on recommendation from a YouTube Guru. This brush costs $21 on Sigma's website but I bought it during a sale. I love this brush! I don't usually apply a full face of foundation. I usually just apply a light foundation or a BB cream under my eyes and around my nose to even out my skin tone. This brush is perfect for that. It blends the foundation beautifully into my skin and it doesn't leave any brush strokes or marks. There is also no fall out whatsoever from this brush.