You build from below ground up; lots of pipes, cables etc are underground and need sorting first(ish).
Surprisingly many older utility works are not properly recorded on plans etc and can be the source of much added expense and delays.
It'll be for a ground survey. You might dig trial pits for a soil survey. Utilities locating as suggested. Good reasons you need excavators on sight for a large development like this at an early stage. Any contractor will quite likely do their own survey rather than risk delay at a later stage. Other have given more detailed reasons. Just good to be chatting like this at last.