Er, I think you've entirely missed the point here mikay.
EVERY operating system needs a boot loader/manager. Windows, by default, doesn't show any options when it's starts. You can configure windows 2k/XP (possibly nt as well?) to add options to the boot menu (eg. mine says: Windows, Recovery Console, or Linux).
The whole point of the 'manager' boot manager is to allow for choice.
There are 2 popular boot loaders for linux - lilo and grub. Lilo and grub both DO give you boot options by default (such as a normal boot, booting a kernel off a disk, etc). It's a lot more likely you will need the functions a boot manager provides under linux. If you recompile your kernel with different modules, or upgrade it, then you want to keep the option to boot the old one as WELL as the new one in case the new one fails to boot.
On my machine, if I choose linux from the XP boot manager, that then triggers lilo (which resides on my master linux partition) which then gives me more boot options which are specific to my linux installation.
This is not the standard way of doing it. Most people will use lilo/grub to boot windows (cutting out the options part of the windows boot manager) but linux is very much my "secondary" operating system (it's only there for aspects of my uni course), so if I ever want to remove it it's less hassle to just erase the partitions and remove the entry from the windows boot manager than it is to uninstall lilo properly and then try and coax the XP boot manager back into action.
I hope thats shown you some examples of why they are necessary.