What makes a simple web tool actually useful

I’ve been experimenting with small web tools lately, and one thing I’ve noticed is that people seem to prefer tools that do one simple task quickly rather than complicated platforms.

I recently built a small collection of name-related tools that follows this idea — combining names, generating couple-name ideas, baby-name combinations, usernames, and similar things.

I’d be interested to know what makes you personally trust a small web tool: simplicity, speed, design, privacy, no signup, or something else?

What features make you come back to a simple online tool?