Its called "stoppage time" because match play is stopped for injury time etc. Its added on at the end so that the total match playing time is equal to 90 minutes. Players & coaches have a pretty good idea how much time will be added and irs displayed on the board at the substitution sideline at professional level. If stopage for injury etc for 2 min, then again for 1 minute they know ~3 minutes will be added at end of that half.