Case study: You have a PC connected to BT OpenWoe which you use to share your Internet connection to say two other computers on your home network - i.e. your Laptop connected via wireless and maybe your kid's computer.
All three try to load a web page at once. Oops - BT Openwoe's system only allows TWO web accesses at once.
You want to have multiple web browser windows? Sorry - can only make two web connections at once.
Want to download a big file over http? (i.e. the WHOLE F***ING POINT OF BROADBAND) oops - this counts as a connection. Download two files at once and you just wiped out your web access (for ALL of your computers) until one finishes.