Sandbags are inexpensive and provide trenches and Field HQs with decent protection from explosions. These can be a good addition to trenches, or mortar pits. if you want to give your guys better protection from explosions, but they aren't very good against bullets. You will hear a distinctive ricochet sound if a bullet hits the sandbags.
During World War II, sandbags were also used as extemporized "soft armor" on American tanks, to help defeat German anti-tank rounds,but were ineffective.