When using a slave strobe, you may need to experiment a bit to get the right balance of light for your closeup and macro shots. Slave strobes can be used for special effects, such as backlighting behind a sea fan or illuminating the inside of a shipwreck. If you use two balanced strobes, connected by a sync cord, balancing the light is simpler. If you want a strong highlight from the right, you move the left strobe further away from the subject. This is more intuitive than using a slave strobe, but the cost is higher, and the cords prevent you from using your second strobe for special effects.
Position is important.
A flash can be positioned to simplify the picture. For this shot of a crayfish launching up from the silty bottom of Scofield Reservoir, I wanted as little of the water, algae flakes, and bottom as I could get. So I positioned the strobes high, aiming down towards the subject. This catches the brown crayfish away from the brown mud, and eliminates background details. Get the strobe away from the camera, then aim your strobe so the EDGE of the strobe's output just covers your subject.