Current User’s Startup Folder location:
C:\Users\<username>\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup

All Users’ Start Folder location:
C:\ProgramData\Microsoft\Windows\Start Menu\Programs\StartUp

AppData & ProgramData folder are hidden, needs to enable Show hidden files, folders and drives

Another faster access method is:
1) Press Windows Key and R key
2) type:
shell:common startup (for All users)
shell:startup (for current users)
3) Click OK

Reason to access Startup Folder:
After Windows loaded, it will load application shortcuts found in Startup folder.
For Example, Outlook. Instead of clicking Outlook after Windows loaded, Outlook shortcut in Startup Folder will auto-load.