Compatibility with legacy programs. You've no idea how old some of the programs some businesses use. Also, those wee cheap atom tablets
You need theinstaller for 64 bit windows.
You either need to download Windows 10 from a PC running 64 bit windows (it checks what you have and downloads the same as you currently have) or use a Windows XP machine or Linux or a Mac and you get the option to choose which version of windows to download.