if you have for example 10 cars its not easy to remembre it and when you are in combat there is no time to check car list and find id then spawn it , in contrary if you shortcut id to specific name you can easily remembre it and spawn it directly
examples :
/getcar heli , /getcar pcj , /getcar pack ,
Even if you remember id you will not be able to know which vehicle so you will have to try until you find what you are looking for