People buy expensive manual-focus lenses when manual focus itself is part of the benefit, or when the lens offers other qualities they value more than autofocus.

Common reasons include:

• Better manual-focus handling: longer focus throw, wider focus ring, smoother/damped feel, and more useful distance or depth-of-field markings.
• More precise focus control for deliberate work, especially when autofocus can’t choose the exact plane you want.
• Optical or rendering preferences: some manual-focus lenses are chosen for sharpness, bokeh, or a particular look.
• Size, weight, and build: without AF hardware, a lens may be simpler, lighter, or very solidly built.
• Reliability/simplicity: no AF motor means one less mechanism to fail.
• Value allocation: if you don’t need autofocus, you may prefer to spend more of the budget on the optics rather than AF features.

Also, not every desirable lens design is available with autofocus, and even when it is, the AF version may cost more. So the appeal is often a mix of focusing experience, image character, build quality, and priorities—not just the absence of autofocus.