Be careful when you say “manual”.
You could mean several entirely unrelated things, including:
- Manual exposure (that’s the “M” on the dial!)
- Manual focus
- Manual flash power
- Manual focus point selection
Assuming you mean the first one, when do I use manual exposure mode?
- When using flash indoors, always.
- When I want full control (such as when shooting macro or art).
- When I want consistency, such as when shooting a panorama or snow scenes.
- When I am shooting impossible-to-meter subjects like fireworks.
- When I am shooting important-to-get right subjects like product or food.
- When the light varies constantly and I want my exposures to be stable.
Hope that helps.