The study behind the café
Mehta, Zhu and Cheema (2012) tested creative tasks under low, moderate and high ambient noise. Moderate noise, around the level of a busy café, produced better results than a quiet room; loud noise made things worse. The proposed reason is that a little difficulty processing the environment pushes thinking toward more abstract, wider connections.
Why the chatter does not hurt
Speech is disruptive when you can follow it. Café murmur is many voices at once, at a distance, blended into a wash. The masking effect is there; the semantic hook is not. If a single clear voice cuts through, that is the sound to cover, and brown noise underneath helps.
Best for
Writing, planning, and anything where a blank quiet room feels heavier than it should. Less good for arithmetic and detailed editing, where most people prefer rain or brown noise.
Mehta, Zhu & Cheema (2012), "Is Noise Always Bad? Exploring the Effects of Ambient Noise on Creative Cognition", Journal of Consumer Research, 39(4).