Spirit form is a state that Servants can take on at will. While in spirit form, Servants have very low capacity to affect the physical world. Accordingly, Servants take on a physical form to engage in combat, as they would otherwise suffer a significant disadvantage. Notably, Saber summoned by Emiya Kiritsugu cannot enter Spirit Form for a “specific reason.”

Fate/Zero Animation Guide I: Glossary of the holy grail war I