Gloss and semi-gloss reflect light more directly, which can make a colour look deeper and more saturated in good light but also shows every surface imperfection and can produce noticeable glare under strong or direct lighting.
A gloss film is smooth at the scale of the wavelength of light, so most of the light reflects in a single direction rather than scattering. The result is a specular highlight — a mirror image of whatever light source is in front of it — sitting on top of the colour, and a colour that looks deeper because less light is being diffused back out of the film.
Matters most on trim, doors, banisters and radiators: small, frequently touched surfaces where the durability is genuinely useful and the reflection reads as crispness. Matters least — and is usually actively wrong — on large wall areas and on any old, uneven joinery, where the reflection maps every flaw.
Applying gloss over woodwork that has not been properly filled and sanded. Gloss does not hide preparation; it advertises it, and every dent and brush mark becomes more visible than it was before painting.
Practical advice
Gloss is a traditional choice for skirting boards, doors and trim where a crisp, durable, easily-wiped finish is wanted, but is rarely used on whole walls because of glare and imperfection visibility at that scale.
Always confirm with a physical sample in the actual finish before committing to a large surface — see methodology.