AM4 motherboards with B450 chipsets are a cheaper alternative to those with X470 chipsets. It’s the equivalent to B350, which was almost always good enough chipset choice and which became the default recommendation for Ryzen builds. B450 chipsets now are in the same position, with one difference: Out of the box compatibility with the Ryzen 2000 series.
They are a welcome addition to the market and a necessary one for AMD, since Intel’s Core i5-8400 together with a B360 board was until now often the better choice over a Ryzen 5 2600 and X470, the Ryzen combination was just too expensive and the i5-8400 too strong in comparison. Now, with B450 available to make AMD builds cheaper, and together with the last price drop of the processor itself, the Ryzen 5 2600 is an attractive option again.