Your phone's GPS would drift 10 km a day. So engineers built the clocks to run slow.

Uncorrected, your GPS would place you 10 km from where you actually stand, every day.: You are here · GPS says here · ~10 km · +38 µs/day

GPS works by timing radio signals from satellites. But those satellites race along at ~14,000 km/h (so their clocks tick slower, per special relativity) while orbiting where gravity is weaker (so their clocks tick faster, per general relativity). The net: the clocks gain about 38 microseconds a day. Left alone, that compounds into roughly 10 km of position drift daily. So each clock is built to run slow by exactly that much before launch.

Relativity isn't abstract: it's pre-loaded into the GPS in your pocket, or the map lies.