After rain, parts of the yard hold water for hours or days. On clay-heavy blocks this is common because the soil barely absorbs water. A French drain gives that surface water somewhere to go instead of sitting and turning the lawn to mud.
A subfloor that smells damp, or weep holes that sit below soil level, points to water collecting against or under the building. Left alone this leads to rising damp, timber rot and the right conditions for termites. A French drain along the affected side intercepts the water before it reaches the structure.
Stand outside during heavy rain and watch where the water goes. If the ground falls toward the building, common when a neighbour's block sits higher, or after landscaping changed the levels, you have a drainage problem a French drain is well suited to fix.
White powdery deposits (efflorescence) on brick or render, staining near the floor, or cracking that tracks with wet weather all suggest moisture moving through the wall. Drainage is usually the upstream cause worth ruling out first.