Studs are pre-cut to standard lengths. This makes buying easy and reduces wastage.  While designing a structure, whether it is a standard garage or a large house, it is best to incorporate the standard structural lumber sizes into the room or building dimensions. For example, some popular pre-cut lengths for 2X4 (two by four) studs are 6, 8, 10, 12 feet.  This means that you can avoid wastage if your garage is either 8 feet high or 10 feet high. This is also the reason a standard 2 car garage is 24X24.  The standard panel size for sheathing material like drywall, plywood, OSB, etc. is 4’X8’. A standard 2 car garage door is 16’X7’. So it all fits in quite nicely.