as BG has said the X-acto is mostly used for hobbies such as modelmaking, stencil cutting etc which need a finer blade offering much finer control over shapes able to be cut and size of cuts.
Most artists will have an X-acto knife somewhere in their kit, often used to sharpen pencils, cut backing/mounting mattes, cut stencils, carve/whittle small maquettes (models) in timber etc.
The utility boxcutter style knife is mostly only good at cutting in straight lines and for heavier duty materials - thicker cardboard, plastics etc. - and may also be used for whittling and sharpening