Bulbs of this type are cheaper to make than certain other types and are therefore cheaper to purchase. As for the brightness, this light bulb clocks in at 220 lumen.
From Amazon
3.2w Filament Candle - Dimmable - Warm White - Small BC Cap (B15) - Megaman
LED Bulbs
Megaman 3.2W LED B15 SBC Filament Candle Warm White Dimmable - 143641
Big on Electricals
Megaman 3.2W LED B15 SBC Filament Candle Warm White Dimmable - 143641