The Buddha's invitation to us is "to come and see" for ourselves what is true. Investigation is the "activity" of mindfulness. It's the activity of discernment. It illuminates the object. When things are brightly lit, what is already present is then clearly seen and known, and confusion is dissipated.
"EHI PASSIKA" "Some and See" investigation is the active aspect of mindfulness. It brings a clear discernment of our body-mind experience. Investigation is what lights up the truth for us.