Away mode why does the motion sensor have a count down to sound the alarm

For the motion sensor in the away mode why is there a count down and not an instant alarm?I want to have this kind of set up just in case someone is able to come into the home without tripping any of the door and window sensors. I know that once the motion sensor is tripped and the alarm is in away mode, it will start the count down before sounding the alarm but I’m still looking to get an alert regardless. Can you assist?